some clean-up and minor changes in the fvpressure base class

   - fluxes between neighbors are now only calculated from one side
   - implementation now called by asImp_()
   - local indices of entries replaced by enums



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7363 2fb0f335-1f38-0410-981e-8018bf24f1b0