Rethink fluidsystems: gasphase.hh + liquidphase.hh vs 1p.hh
@becker and I think
liquidphase.hh should be "real" fluidsystems. Then
1p.hh is not necessary anymore (deprecate).
liquidphase.hh have duplicate functions for density, viscosity, etc. using either temperature + pressure or fluidState + phaseIdx as input. Is this necessary or should that be simplified to one function. As far as we know, the call to those functions using T+p is only used by 2pimmiscible.hh and 1p.hh.
liquidphase.hhare already real fluidsystems
change call to e.g density functions in