Patch 2 for the alpha release

This patch corrects two major issues with the code:

1. The velocities beyond the sonic point are now consistent with a planetary outflow (previously, it was consistent with an accretion flow beyond the sonic radius).
2. The calculation of the sound speed now allows the user to set an ionization fraction, which in turn changes the mean molecular weight of the atmosphere.

Very first alpha release

This is the very first alpha release of p-winds, which calculates the steady-state distribution of ionized and atomic hydrogen in a planet's upper atmosphere.