Corrected computation of diffusion - adapted it to the way it is done in the...

Corrected computation of diffusion - adapted it to the way it is done in the 2p2c box model. The diffusion coefficients are for molar concentration gradients and should use the correct gradients (of the mole fractions) and the molar density. The computation of the respective variables has been added.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7927 2fb0f335-1f38-0410-981e-8018bf24f1b0