Fix UGGridEntity::partitionTypes method in absence of MPI

When writing the partitionTypes method I forgot to add the special code for the case that MPI is not there. For some strange reason the CI system let me get away with this. This patch adds the missing code.

mentioned in merge request staging/dune-uggrid!165 (merged)

approved this merge request

See passing pipeline https://gitlab.dune-project.org/staging/dune-uggrid/-/pipelines/29175

merged

mentioned in commit ccac9ac0