Multiple small fixes to docstrings and typing

[RondeauG]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • No.
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• This PR does not seem to break the templates.
• HISTORY.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Removed a few outdated imports.
• Typing was added to multiple functions when it was missing.
• Output types were added to multiple functions when it was missing.
• Docstrings was added to multiple functions when it was missing.
• Both str and os.PathLike should be explicitly supported where relevant.
• Minor bugfixes when I saw them.
• Module descriptions were added when missing.
• To make things readable:
  • As much as possible, the type of data in lists was defined.
  • The type of data in dictionaries was not.

Does this PR introduce a breaking change?

• It should not.

Other information:

• Pycharm recommended that I add ABC to DataCatalog/ProjectCatalog. It this right? @aulemahal
• The init and documentation of DataCatalog/ProjectCatalog should be reworked to be easier to understand, but this is out-of-scope for this PR. For example we use *args, but AFAIK there is only one possible input argument, obj.

[Zeitsperre]

Good changes overall. I think the Optional usage needs to be clarified here. Will be good to go afterwards!

EDIT: To reiterate, the Optional serves both the IDE/user and helps with the static type checking that would come from mypy. We don't currently have any projects that are 100% mypy-compliant, but the more we implement now, the easier it will be in the future.

Why use mypy? https://python.plainenglish.io/mypy-your-secret-weapon-for-code-brilliance-a376d15c463c

[Zeitsperre]

Lots of work here. looking good!