sequential solver backends can now also be used for the box models

   - of course the model can not longer run parallel if a sequential
     backend is chosen!
   - removed sequential SuperLU backend from boxlinearsolvers -> backend
     defined in seqsolverbackend.hh



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6322 2fb0f335-1f38-0410-981e-8018bf24f1b0