Move FindPackages for (Par)METIS, SuiteSparse and PTScotch to dune-istl
There are several FindPackages in dune-common that are not used anywhere and also the the dune-common code is not dependent on those packages. Those packages are
METIS and ParMETIS are used in dune-istl and dune-grid, and SuiteSparse is used in dune-istl. Don't knwo where PTScotch is used.
Does it make sense to move the cmake files to the modules where they are actually used?