- mpnc diffusion: remove bug: the liquid phase diffusion was multiplied twice...

- mpnc diffusion: remove bug: the liquid phase diffusion was multiplied twice with the crossectional area: face().normal has already the magnitude of the area it is sitting on.
- actually enable the diffusion module for testing
- new reference solutions


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10113 2fb0f335-1f38-0410-981e-8018bf24f1b0