Rethink isGas(), isLiquid(),... functions in fluidsystems
Some fluidsystems have a
isGas() and a
isLiquid() function and some fluidstates (compositional, nonequilibrium,...) use those to assert that the correct phase is used/assumed.
For any two/three phase system either one of the two options should be sufficient but we do not need both (if we need them at all).
adapt fluidsystems and fluidstates so that we consistently either use
isXY()-type functions if they can be simplified/generalized