Fix Eigen solver backend

The EigenBackendSPD_Base in dune/pdelab/backend/eigen/solvers.hh has a template-template parameter template<class, int> class Solver. In the subclass EigenBackend_SimplicialCholesky_Up this parameter is set to Eigen::SimplicialCholesky which has the signature template <class, int , class> class (link to Eigen doc). The latter fails to compile.

Simply changing the template-template parameter to template <class, int , class> class and using the default value from the Eigen-Documentation for the third parameter (AMDOrdering<...>) in the apply method of EigenBackendSPD_Base seems to fix this problem, but I don't know if this was intended originally and I didn't look closer into the Eigen3 documentation.