correctly convert mole to mass fractions in SettablePhase

also make the densities in the water-nitrogen fluid system
consistent. this makes the results identical to to ones obtained by
the new fluid framework

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7082 2fb0f335-1f38-0410-981e-8018bf24f1b0