What this MR does / why does DuMux need it:
Introduces a 2p2c-specific constraint solver, that explicitly calculates mass fractions for two-phase state, assuming ideal mixtures (as does the generic mpnc constraint solver for two-phase state).
Changes in volumevariables: The new constraint solver is used for two-phase state. In the one-phase state the ComputeFromReferencePhase solver is used, since it does the simplest thing possible for ideal mixtures.
useConstraintSolver is deleted because it's not needed anymore.
Resolves #761
Special notes for your reviewer:
Is it okay to have a new header for the 2p2c-specific constraint solver or should I integrate it as a special case in the generic one? I guess this is not backward compatible (deleted useConstraintSolver)? Do I have to deprecate the entire old volumevariables? @holle : The 2p2c tests parse with the new constraint solver (they did so before with the old, in my opinion slightly less correct/buggy version, too, although that was never tested...). Does it look good to you, too?