h2o-n2 fluid system: use the total pressure to calculate the component enthalpies

this is analogous to commit r12402 of dumux. The current approach was
doing the "compositional consideration" twice. (and the difference
between using the partial pressure for the enthalpy and using the
total one in conjunction with weighting by mass fraction should be
neglectible for "almost ideal" gases, i.e., for low pressure and
temperature scenarios.)