Made diffusion work again: The density at the integration points has not been...

Made diffusion work again: The density at the integration points has not been calculated in the flux variables and was thus 0 --> no diffusive fluxes.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7912 2fb0f335-1f38-0410-981e-8018bf24f1b0