replaced mimetic finite difference implementation of the 2p pressure

equation by the newer implementations from dumux-devel

   - includes gravity terms but no capillary pressure terms!
   - additional implementation for adaptive grids



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11406 2fb0f335-1f38-0410-981e-8018bf24f1b0