Increase documentation quality

[davidrudlstorfer]

This would be the last (famous last words) big meta issue for BeamMe.

With the switch to pytest, a src based layout and the overall switch to .yaml based input the code quality increased by orders of magnitudes.

The last step would be to increase the documentation quality (which 1:1 gets piped into our API docu).

For that we should consider these points:

• Add type hints (feat: add type hinting 4C-multiphysics/fourcipp#54 could be helpful, https://github.com/gilrrei/MonkeyType could be helpful)
• Enforce type hints (Enforce and fix type hinting and update pre-commit hook 4C-multiphysics/fourcipp#80 could be helpful)
• Update/Add docstrings according to Google Style across the entire code base (@isteinbrecher I think we can close Update NURBS functions to the current google docstring style #305 as this is an issue for only one file). I think it would make sense to do them file wise. This way the review and implementation effort is limited)
• agree on one global "standard" for file docstrings (these are the module descriptions within the API docu), sometimes we use "this file defines", "this module defines", ...

@isteinbrecher let me know what you think:)

[isteinbrecher]

I agree with all points! In the long term that is definitely something where we should aim to get to.