Move calculation and data normalisation into a shared library

[Greenheart]

Since calculations e.g. for calculatedTotalEmissions, and data normalisation are common tasks that are complex and error-prone, this would be great to move to a separate shared library which could have proper tests to make sure it covers all edge cases.

Then we could use this logic in the API as well as on the frontend.

This would likely be a good task to pick up after we've migrated to a monorepo as per #490