p-winds

This is a patch to the stable version of p-winds. Here are the changes: Corrected a bug in hydrogen.py where the code passed velocity and radii to parker.average_molecular_weight() in units of values at the sonic radius instead of km/s and Jupiter radii. This correction has only a small impact to the resulting models. Improved rendering of the documentation in ReadTheDocs. Some small updates in the test suites.If you use this software, please cite it as below