extended functionality of new structured decoupled models

   - added add... functions to 2p flux data
   - removed bug in 2p adaptive fv model (TPFA) -> velocity is now again
     calculated by the pressure model to make sure all velocities are
     availabe in the transport model when needed
   - some minor changes/corrections



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7511 2fb0f335-1f38-0410-981e-8018bf24f1b0