Publish to PyPI and Coveralls

[ElliottKasoar]

Resolves #3, #4, #5, #6, #7, #8

Again, heavily based on the equivalent changes on aiida-mlip.

This repository is currently being tracked by Coveralls, and the secret has been added for PyPI, so hopefully everything should work as is.

Note, these are resolved in terms of infrastructure. I'll open separate issues for:

• Actual documentation
  • Setup, dev guide etc., not just for code which we can add as we develop
• Add nox (see in-progress work on aiida-mlip) or tox (see janus) for better testing