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 isGas() or isLiquid()