some adaptions to the stuff decided today

- GenericFluidState -> NonEquilibriumFluidState (I don't remember the new name of EquilibriumFluidState)
- removed updateAverageMolarMass from NonEquilibriumFluidState and EquilibriumFluidState
- removed sum{Mole|Mass}Fractions from NonEquilibriumFluidState and EquilibriumFluidState
- all fluid states and systems now use enthalpy instead of internal energy

On a completely different front, this also makes the MpNc model
compile with energy enabled, although the implementation is still
incorrect. (as long as nobody is unhappy about it I won't change
this in this branch!)

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7059 2fb0f335-1f38-0410-981e-8018bf24f1b0