The model can be used with either mole or mass fractions. The property

useMoles has to be set in the problem file and the boundary conditions 
have to be choosen accordingly.

The model still uses MASS fractions by default.

The documentation is changed and extended accordingly.

reviewed by kathinka


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11529 2fb0f335-1f38-0410-981e-8018bf24f1b0