Thermally-perfect gas model

[cwentland0]

Need to add a thermally-perfect gas gas_model, with all the necessary instance methods which are already implemented within CaloricallyPerfectGas. This will require new routines for loading the McBride transport and thermodynamic property polynomial tables. Routines for the derivatives and density and enthalpy can be ripped from GEMS.

Note, this will make solving in conservative variables (dual_time = False) particularly tricky, as backing out the stagnation enthalpy without access to temperature will require an iterative procedure. Nick had trouble with this, but such a routine already exists in GEMS. It should be easy enough to copy it from there.