[parallel] Load balancing for UG with PTScotch

There is a helper in Dune that shows how to load balance a UGGrid with ParMETIS at https://gitlab.dune-project.org/core/dune-grid/blob/master/dune/grid/utility/parmetisgridpartitioner.hh

Background is that the native load balancer in UGGrid seems to produce quite bad partitions. AluGrid's partitions are based on ZOLTAN/ParMETIS/Scotch and seem to be much better.

Since ParMETIS has a weird license (education-only), it would be be nice to have the same feature using (PT)Scotch which is (licensed under CeCILL - compatible with GNU GPL). Scotch offers similar capabilities and we already have a scotch backend for matrix reordering.