Rewrite entity maps to be MeshTags

[jorgensd]

Describe new/missing feature

Instead of having entity_maps as a dictionary, it should be a dolfinx::mesh::MeshTags object,
where the topology of the MeshTag is the mesh we are mapping to, the indices the entities in the input mesh topology, while the values are the entity number in the mesh we are mapping from.

This means that inverting the map is as easy as switching order of indices and values (and attaching the correct mesh topology), and we avoid storing -1 indices (that currently are resulting in segfaults whenever there are issues with user input, making it very hard to debug).

We would also no longer need a dictionary for the maps, as the topology is stored as part of the MeshTag, and we can create a topology -> list_index_in_input_entity_tags.

Internally in DOLFINx, we could unravel these to avoid calling find many times (similar to what we do with Dirichlet BC dofs), but then with explicit handling of the negative/non-existent entities.

Also note that creating the "parent_to_sub_mesh" map is a simpler operation than its inverse (as we do not have to sort the input entities to entity_map.

Suggested user interface

mesh = .....
sub_indices = ....
submesh, sub_to_parent, _, _ = dolfinx.mesh.create_submesh(mesh, dim, entities)
entity_map = dolfinx.mesh.meshtags(submesh, dim, np.arange(len(sub_to_parent), dtype=np.int32), sub_to_parent)

# Given a parent cell `c` find cell in submesh
sub_cell = entity_map.find(c)

# Invert map
parent_sort = np.argsort(sub_to_parent)
sub_cells = np.arange(len(sub_to_parent), dtype=np.int32)[parent_sort]
inverted_map = dolfinx.mesh.meshtags(mesh, dim, sub_to_parent[parent_sort], sub_cells)

# Given a child cell `d` find parent_cell
p_cell = inverted_map.find(d)