removal of incomplete interface for basing mass transfer on chemical...

removal of incomplete interface for basing mass transfer on chemical potentials in stead of mole fractions. 
Future work ...

reviwed by Christoph

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12744 2fb0f335-1f38-0410-981e-8018bf24f1b0