Python docs improvements

[bluescarni]

Keep track of useful improvements to the Python documentation:

• document special members such as __len__, __getitem__, __copy__, __deepcopy__ that are implemented in some classes (e.g., island, archipelago, population, etc.). This can be done by adding the sphinx directive :special-members: to the autoclass directive, but then this brings in the __init__ documentation which is repeated in the class description. Need to find a way to make it all look consistent and nice.

[bluescarni]

moved to #53