Support mixed topology meshes

[garth-wells]

Support mixed topology (different cell) meshes.

See https://github.com/FEniCS/dolfinx/projects/8

[RemDelaporteMathurin]

Hi @garth-wells , are mixed meshes still planned or not at all?

[garth-wells]

Hi @garth-wells , are mixed meshes still planned or not at all?

It's underway in https://github.com/FEniCS/dolfinx/tree/mixed-topology-hackathon-23.

[RemDelaporteMathurin]

I see #2597 started implementing mixed topology meshes! Is the project board still up to date?

[garth-wells]

Start was made, expecting to continue ~end July.