First steps to unify seqsolverbackend and boxlinearsolver.

Created new LinearSolver type tag and corresponding properties. 
BoxModel and DecoupledModel inherit these, which should be ok, since we
most likely will never have one of those models without a linear solver. 


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6308 2fb0f335-1f38-0410-981e-8018bf24f1b0