Generalise topology to take multiple cell types

[chrisrichardson]

Adds a new create_topology() method that takes a list of cell_types, cells, ghost_owners and original_cell_index, instead of single values for each.
The resulting topology now has index_maps and connectivity between each type, and the types of the mesh are stored in entity_types (instead of cell_type).
To access the extra tables of connectivity and index_maps, we now need to supply an index within each dimension: e.g. connectivity((3, 0), (2, 1)) gets the connectivity from cell_type 0 to facet_type 1 (in a 3D mesh).
Some tests are added.

Behaviour for single cell_type mesh is unaffected.

updated