Contrib closes #23 (#251)

* add contribution guide

* add coding style

---------

Co-authored-by: ElliottKasoar <45317199+ElliottKasoar@users.noreply.github.com>
Co-authored-by: Jacob Wilkins <46597752+oerc0122@users.noreply.github.com>

coding_style.md

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+# Coding style
+
+We adhere to [pep8](https://peps.python.org/pep-0008/), which is automatically enforced via a pre-commit in the CI.
+
+Please check your code manually before committing. Suggestions for how to perform these checks, as well as additional information about the checks performed, can be found in the [developer guide](https://stfc.github.io/janus-core/developer_guide/get_started.html).

contributing.md

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+# Contribution workflow and the review process
+
+This document outlines the best practice, using git and CI, which
+**must** be followed for all contributions to janus-core. Also contained
+are instructions and tips for managing your fork of the project which
+will help keep merges clean and avoid many headaches.
+
+## Golden rules
+
+In brief the rules for contribution are as follows:
+
+-   Follow the branch, fix, merge model, from your own fork or fork/branch model.
+-   An issue must be created for every piece of work (bug, feature,
+    *etc.*)
+-   Pull/merge requests will not be accepted without a review
+-   New features must have a test
+-   All tests must pass, no regressions may be merged
+
+## Issues
+
+### Using issues
+
+-   Open an issue for each piece of work done.
+-   Open issues for design discussions. The answers may be important for
+    newer members.
+-   When adding features, *e.g.* a new MLIP, use comments to provide succinct reports on progress.
+
+### Labels
+
+Labels may be assigned to issues to help classify them. Examples include:
+
+-   Bug: something is not working as expected. We typically aim to fix these ASAP
+-   Enhancement: adding or improving on features
+-   Testing: adding or enhancing unit testing or CI
+-   Documentation: adding or enhancing documentation
+-   Question: Something is unclear
+
+## Review
+
+All merge requests will be reviewed to ensure the integrity of the code.
+
+The reviewer(s) have the following responsibilities:
+
+- Ensuring all contribution rules have been followed
+- Ensuring the [coding style](./coding_style.md) is adhered to
+- Only accepting a merge if all tests have passed
+- Using the comments system to request changes for the submitter to make
+
+### Enforcing style
+
+GitHub Actions will automatically run a pre-commit and enforce the coding style.
+To reduce the number of CI failures, please run the pre-commit locally before
+you push to your repository.
+
+```sh
+   pre-commit run --all-files
+```
+
+## Using git for development
+
+The GitHub instance hosts an *upstream* repository, which we will refer to
+as *upstream*. Contributors will work on their own personal copies of the
+repository by creating *forks*. This allows us to keep *upstream* clean
+(one commit per merge request, if possible, otherwise commits
+should be squashed before merging. All commits should pass tests.)
+while users may work on their own *fork*, creating commits and changing
+the code as they see fit. Only in exceptional circumstances are branches allowed in *upstream*.
+Please discuss in the zulip channel before you push any.
+
+The *upstream* repository may be cloned as follows,
+
+``` sh
+git clone git@github.com:stfc/janus-core.git janus-core-stfc
+```
+
+A *fork* is created using the web UI. It may then be cloned for a user
+called 'username' as follows,
+
+``` sh
+git clone git@github.com:username/janus-core.git janus-core-username
+```
+
+### Branch, fix, merge model:
+
+All work should follow the workflow of branch, fix, merge. Let us assume
+you have an issue with your code which needs to be fixed.
+
+#### Step 1: Branch from your fork
+
+Create a new branch for the issue on the dashboard of your fork, we will
+assume the branch is called 'issueXYZ'. Clone the branch,
+
+``` sh
+$ git clone -b issueXYZ --single-branch git@github.com:username/janus-core.git janus-issueXYZ
+```
+
+Alternatively you can create the branch in the cli using
+
+``` sh
+# clone the repository, if you already have a local repository this is not nessecary
+$ git clone git@github.com:username/janus-core.git janus-issueXYZ
+$ pushd janus-issueXYZ
+# create and checkout a new branch (this is equivilent to git branch followed by git checkout)
+$ git checkout -b issueXYZ
+# create a remote tracking branch for you to push your changes to
+$ git push -u origin issueXYZ
+```
+
+#### Step 2: Fix the issue and commit your changes
+
+Fix whatever is wrong. Use `git status` to see which files you have
+changed and prepare a commit.
+
+``` sh
+# stage changes
+$ git add <filename|folder>   # to add the new things
+# commit the changes with a clear and brief message
+$ git commit -m "<commit message>"
+# push the commit to origin
+$ git push
+```
+
+#### Step 3a: Merge your branch into upstream
+
+Follow the provided link after the push and continue to the web interface.
+Add any relevant labels or milestones and
+assign a reviewer. Compare the code and if you are happy click to submit your pull request.
+
+After the pull request has been submitted, tests will be run and your
+reviewer will be notified.
+
+#### Step 3b: Finalising the merge
+
+If all is OK with the commit your reviewer may set the request to be
+merged once all tests pass. Otherwise the reviewer may open discussions
+using the GitHub comment system to point out issues that may need to be
+addressed before the commit can be merged.
+
+If changes need to be made you may make more commits onto your branch.
+When you push your branch to your *fork* the pull request will be
+automatically updated to use the latest commit. Reply to the discussions
+to indicate when and how they have been addressed.
+
+If your branch has become out of sync with *upstream* then conflicts may
+arise. Sometimes these cannot be automatically resolved and you will
+need to resolve them by hand. GitHub provides instructions for this, or
+you can follow this routine,
+
+``` sh
+# add upstream as a remote if you have not already
+$ git remote add upstream git@github.com:stfc/janus-core.git
+# get the changes to upstream since you started working on your issue
+$ git fetch upstream
+# merge these changes into your branch (assuming you want to merge into the main branch on upstream)
+$ git merge upstream/main
+# resolve any conflicts
+# push to your fork
+$ git push
+```
+
+Alternatively you may use rebase which will replay the changes you made
+in your branch on top of *upstream/main*. However, be sure you understand
+the differences between merge and rebase
+
+``` sh
+# add upstream as a remote if you have not already
+$ git remote add upstream git@github.com:stfc/janus-core.git
+# get the changes to upstream since you started working on your issue
+$ git fetch upstream
+# merge these changes into your branch (assuming you want to merge into the main branch on upstream)
+$ git rebase -i upstream/main
+# resolve any conflicts
+# push to your fork
+$ git push
+```
+
+
+### Advanced git
+
+#### Keeping your fork in sync with project
+
+By adding two remotes, one for *upstream* and one for your *fork* it is
+possible to keep your *fork* in sync with *upstream*. This will greatly
+simplify merge requests. GitHub also offers a sync functionality in their
+web UI that achieves the same.
+
+``` sh
+# clone your fork
+$ git clone git@github.com:username/janus-core.git janus-core-username
+pushd janus-core-username
+# add a remote for upstream
+$ git remote add upstream git@github.com:stfc/janus-core.git
+```
+
+These commands need to be done only once. `git remote -v` shall show you
+the origin and project fetch and push links
+
+``` sh
+$ git remote -v
+origin  git@github.com:username/janus-core.git (fetch)
+origin  git@github.com:username/janus-core.git (push)
+upstream git@github.com:stfc/janus-core.git (fetch)
+upstream git@github.com:stfc/janus-core.git (push)
+```
+
+When you need to sync your *fork* with *upstream*, do the following,
+
+``` sh
+# get the latest commits from upstream
+$ git fetch upstream
+# ensure you are in the main branch of your fork
+$ git checkout main
+# merge your main branch into the main branch of upstream
+$ git merge upstream/main
+# push these changes back to the remote of your fork (origin)
+$ git push
+```
+
+of course one can use a similar process to merge any other branch or
+available projects.
+
+#### Rebasing commits
+
+When working on an issue you may use multiple commits. When you are
+ready to create a merge request, you should squash your changes into one
+commit in order to keep *upstream* clean. This is most easily achieved with
+an interactive rebase.
+
+Assuming you have made five commits,
+
+``` sh
+# rebase your branch five commits before HEAD i.e. where your branch originally diverged
+$ git rebase -i HEAD~5
+# follow the instructions. 'pick' the first commit then 'sqaush' or 'fixup' the rest.
+# You should now be left with a single commit containing all your changes
+# Push your commmit to the remote, use --force-with-lease if you have already pushed this branch to
+# 'rewrite history'
+$ git push origin branchname --force-with-lease
+```
+
+using force is a powerful and dangerous option, use it only if you know
+150% nobody else touched that branch.
+
+#### Cleaning stale branches
+
+Deleting branches from the web interface will get rid of the remotes and
+not of your local copies. The local branches left behind are called
+stale branches. To get rid of them
+
+``` sh
+$ git remote prune origin
+```
+
+To delete a local branch
+
+``` sh
+$ git branch -d localBranch
+```
+
+if unmerged commits exists but you still want to delete use
+
+``` sh
+$ git branch -D localBranch
+```
+
+To delete a remote branch on the remote *origin* use
+
+``` sh
+$ git push -d origin remoteBranch
+```
+
+## Code Coverage
+
+Ensure that any code you add does not reduce the code coverage in a meaningful way. Reviewers may insist on new test to be added,
+please cooperate.