Conveniently set ideal gas EOS when not scale free

[brryan]

Right now we have to set the specific heat in the input parameter file to use an ideal gas EOS. This is yucky in cgs units because it's something like kb/mp, which one has to enter manually (and without guarantee that those values of those constants are in agreement with what phoebus uses internally).

It would be nice to have another option for setting ideal gas EOSs, maybe that allow for specifying mean molecular weight in units of mp and the adiabatic index.