CHOLMOD: disable METIS support for large matrices

See "metis_dswitch" and "metis_nswitch" in Cholmod documentation:

There are (random) segmentation faults in the combination CHOLMOD + METIS for large matrices!

In order to guaratee a stable algorithm the usage of METIS is disabled for matrices larger than 3000 (default value in metis_nswitch)

See, e.g. https://git.imp.fu-berlin.de/agnumpde/dune-solvers/-/merge_requests/36/

for failing CI problem.