clean up docstring formatting and type hints

[aidanjgriffiths]

Tasks

• Make docstring format consistent with the Google style (Napoleon) with 72 character limits per line.
• Add section to contributing about using the Napoleon format.
• Fix mypy type-hint issues. (spun-out into Add mypy support for type hints #27 )

This PR also addresses the relative imports in the scores.probability module

[aidanjgriffiths]

@nicholasloveday and @tennlee, this MR has been rebased with main and is ready to review.
I will copy spin out the remaining tasks into seperate MRs since I don't really have the capacity to address them right now.

[tennlee]

Thank you very much for all this work. I think we should talk about splitting up the MR into smaller parts, as I think it might take a while to work through the discussion about the right constraints for the type hinting, whereas the other parts of the MR could go ahead without much discussion.

[nicholasloveday]

@aidanjgriffiths some functions specify the type in the docstring (e.g., mae), while other functions don't (e.g., mse).
Can you make this consistent? I think the type is not needed in the docstring if you are using typehints.

[tennlee]

Thanks very much for this work! I've commented on a few parts which need a little more tweaking before it's done.

[tennlee]

I have directly discussed readiness of this merge with the author, so I am manually overriding the change request status. Thanks very much for the work!