Enforce constraints in fluid state or algorithm?
Currently only one mass fraction has to be set for the compositional fluid state, the other one is internally deduced from that. This only works for two component models. In general: is it a good idea that the fluid state enforces such constraints or wouldn't it be better if the algorithm took care of that? @timok suggests that the fluid state could have a function like checkSumMoleFractions(Scalar value=1.0) to be able to assert the constraint before an algorithm that has that assumption about a fluid state.