Hard-coded Sutherland model variables
In the current release, the value of `C' is hard-coded. See: https://gitlab.nektar.info/nektar/nektar/blob/master/solvers/CompressibleFlowSolver/Misc/VariableConverter.cpp#L179
In general, C = 110./288.15 = 0.38175 is incorrect. For example, for Argon: C = 144./Tref