Documentation - minor changes to docstrings, remove unused imports and make class __init__ visible on sphinx

Small change to conf.py that handles the generation of our documentation on read the docs. Adding autoapi_python_class_content = "both" ensures that the docstring defined inside the __init__ is also shown on readthedocs. Also entries for __str__ and other __foo methods are not shown anymore.

before:

after:

This closes #314.

Besides that, I made some minor changes to docstrings and removed some unnecessary unused imports from some files.