Fixed Cp,Cv, gamma calculation bug in cpropep

There was an underlying bug in cpropep that we fixed here. It effected Cp, Cv, and gamma calculation in equilibrium states but not rocket performance.

Added add_propellants_by_mass to equilibrium.py

v0.1.2

Modified pypi deployment password

Updated propellant.dat with NIST Propane

Added a PROPANE (NIST) entry with the heat of formation published by NIST.