Use numpydoc format for doc-strings

I mentioned this quite early, so maybe it wasn't completely clear what the consequences were...

This holds especially for the parameter format.

https://numpydoc.readthedocs.io/en/latest/format.html

added Documentation label

It may be that https://github.com/cbillingham/docconvert can solve it (and I may try...).

assigned to @johku144

I have skipped updating the documentation for Flag as of now since it will probably be modified a lot or even removed (Issues #3 (closed) and #24 (closed)).

mentioned in merge request !27 (merged)

mentioned in merge request !32 (merged)