Allow energy/enthalpy shift in IdealGasFluidProperties

[joshuahansel]

Motivation

Ideal gas fluid properties are useful for many applications. For 2-phase applications, they need to be able to shift specific internal energy and specific enthalpy to be consistent with the liquid phase. This is essential for computing latent heats correctly, for example. A shift can be added to the specific internal energy definition without breaking the fundamental assumptions of the ideal gas law:

e = cv * T + e_ref

Design

Add a new optional parameter e_ref defaulting to zero. This is the same as q in StiffenedGasFluidProperties but renamed for greater clarity.

Impact

No impact to existing users.