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.