added additional preconditioner-solver backends

   - the linear solver part is a little confusing at the moment as there
     are various files including various  backends for different
     models, ... (maybe this could be unified in the future)



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6139 2fb0f335-1f38-0410-981e-8018bf24f1b0