Generalize enthalpy energy/temperature calcs in calc_state_from_cons/prim() to each GasModel

[cwentland0]

Right now, calc_state_from_cons() and calc_state_from_prim() are very specific to the CaloricallyPerfectGas model, particularly the stagnation enthalpy calculations implicit in the energy and temperature calculations. This should be generalized to a given gas model, as this is very different for the upcoming TPG model (#18).