MpNc test problem: make the liquid phase consist of pure water instead of pure nitrogen

oops.

if "useSimpleRelations" in the H2O-N2 fluid system is set to true and
the density of the phases is not normalized by sum of the mole
fractions, the results are identical with the ones on trunk.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7081 2fb0f335-1f38-0410-981e-8018bf24f1b0