Docstrings for the python bindings

[mjlarson]

At the moment, the existing python interface is usable, but doesn't have great documentation. I often use jupyter's Shift+Tab keybindings to bring up docstrings for unfamiliar functions in order to better understand the purpose of each method. For NuSQuIDS, this currently brings up the default description showing the C++ signature, but not much less. For example, this, when trying to understand the Earth class:

> import nuSQuIDS as nsq
> nsq.Earth?

Docstring:      <no docstring>
Init docstring:
__init__( (object)arg1) -> None :

    C++ signature :
        void __init__(_object*)

__init__( (object)arg1, (str)arg2) -> None :

    C++ signature :
        void __init__(_object*,std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)
File:           /opt/conda/lib/python3.8/site-packages/nuSQuIDS.so
Type:           class
Subclasses:     

Adding python docstrings would help new users to understand the interface better and increase usability.