Non-deterministic partitioning

[fsimonis]

I ran into multiple issues regarding the partitioning in #212.

METIS_PartMeshNodal turns out to be non-deterministic by default. I fixed the seed in #212, which is enough for the CI, but not for local runs as versions 5.1.0 on Ubuntu 22.04 LTS and 5.2.1 on Archlinux lead to different results for identical inputs and seeds.

The kmean partitioning currently doesn't use a fixed seed, and we will likely run into issues with different versions too.

We need to find a reliable way to get useful =! uniform clustering for the CI.