Submesh support

Things to think about after discussions with @ottarph:

- If we create a submesh and a function on the submesh, this is a stand-alone grid, this can be stored.
- If we store both mesh and submesh as two separate checkpoints, we need to use the relations between parent and submesh cells to ensure that the submesh is distributed with the same ownership as the parent mesh. Probably need to store the entity maps as well, or reconstruct them from the input global indices.