Enable METIS by default
Summary
METIS is disabled by default. It must be additionally enabled in ALUGrid setting -DALUGRID_DISABLE_MMETIS=0
. This MR inverts the logic. By default METIS is enabled and can be used, but one can disable it for ALUGrid by setting the corresponding variable.
Enabling METIS leads to additional tests that are enabled. In the CI system there is only scotch-metis available but not the full METIS library. This leads to errors. Thus, for the CI, METIS is disabled again.