Implementation of CHOLMOD Solver
This class wrappes the dune-istl CHOLMOD method for dune-solvers module.
It is basically a copy of UMFPack solver.
There are problems with some METIS implementations (especially LibScotchMETIS) causing random segfaults. Therefore the usage of METIS is disabled for matrices larger than 3000 rows for now (whichs seems stable), see https://gitlab.inria.fr/scotch/scotch/issues/8