Format docstrings so they pass `pydocstyle`'s criteria

[JackKelly]

Detailed Description

We plan to use pydocstyle to automatically check that our docstrings conform to a standard format. It would be amazing to have help tweaking our existing docstrings to conform!

pydocstyle's list of criteria is here: http://www.pydocstyle.org/en/stable/error_codes.html

Here's the Pull Request (and discussion) about enabling pydocstyle for nowcasting_utils: https://github.com/openclimatefix/nowcasting_utils/pull/23

[jacobbieker]

The PR in MetNet showing the errors is here #2