More rigorous fluid properties

[andr1976]

Include more rigorous fluid model that gives density etc. as a function of both temperature and pressure (i.e. 2D-interpolation) in order to model fluids like CO2 going from gas -> super critical -> dense/super critical liquid where the density and compressibility is far from linear. The current approach works quite well for hydrogen (at not too high pressures), but this enhancement would really be an awesome feature. Tabulation could be done with CoolProp. Also (but might deteriorate speed even more) a direct call to CoolProp for properties could also be interesting.