unify wettingPhaseIndex() interface

In the CompositionalFluidState, there is the interface

int wettingPhase() const { return wPhaseIdx_; }

With !1684 (merged) merged, all two-phase flow related VolumeVariables implementation introduce:

int wettingPhaseIdx() const { return wPhaseIdx; }

On the other hand, the ImmiscibleFluidState does not provide a function to wetting phase index. We should probably define a unified interface and define one place where this should be stored.

I personally would vote for wettingPhase(), and I would propose to store it in the fluid state, because I recall that for compositional models it was necessary to have it in there because the flash solvers need it.