[partmesh] Enable -o flag with --ogs2metis

This allows to run the workflow out-of-source:

cd my-model-dir
mkdir 20
partmesh -o 20 -i my.vtu --ogs2metis
partmesh -o 20 -n 20 -i my.vtu -- boundary_meshes*.vtu

1. Feature description was added to the changelog