Add mixed topology functionality to mesh partitioning

[chrisrichardson]

Mesh partitioning needs a "dual graph" generated from cell-cell connectivity. The current code assumes a single cell type, but this can be updated: https://github.com/FEniCS/dolfinx/tree/chris/mixed-dual-graph

• [x] build dual graph with mixed topology
• [x] send mixed cells into the partitioner
• [x] distribute cells

[chrisrichardson]

Work in progress in https://github.com/FEniCS/dolfinx/tree/chris/mixed-partitioner2