Check and update docstrings

[greschd]

The docstrings, in particular of PwInputFile and CpInputFile, need to be checked and potentially updated. This is probably easier once #44 is done, so we can directly check the doc output.

An issue that definitely needs solving is that they have a docstring both at the class level and for the __init__. In the documentation, only one of the two will show.

It'd be nice to convert the docstrings to numpy-style while we're at it, see https://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_numpy.html