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 default value for the property UseMoles was missing, this has been added and set to the formulation which uses mass fractions.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7926 2fb0f335-1f38-0410-981e-8018bf24f1b0