Simplify MeshGeometry initialisation

[connorjward]

This seems tractable now that mesh.init is gone. This is part of a wider refactor that I want to do but this seems like a good start.

Key contributions:

• Remove MeshGeometryCargo
• Remove MeshGeometry.__new__, now we just have MeshGeometry.__init__ that we treat normally

The only really ick thing that I have had to do is to do with what happens when we have multiple cell types. The code I've written is not nice but all it is really doing is making the existing situation more explicit. I don't consider that to be something that needs fixing in this PR.

[connorjward]

The only failures now are some quite inscrutable IO ones with errors like:

2	[0] DMPlexSectionLoad() at /__w/firedrake/firedrake/petsc/src/dm/impls/plex/plex.c:2660
2	[0] DMPlexSectionLoad_HDF5_Internal() at /__w/firedrake/firedrake/petsc/src/dm/impls/plex/hdf5/plexhdf5.c:2972
2	[0] ISLoad() at /__w/firedrake/firedrake/petsc/src/vec/is/is/interface/index.c:1673
2	[0] ISLoad_Default() at /__w/firedrake/firedrake/petsc/src/vec/is/utils/isio.c:113
2	[0] ISLoad_HDF5() at /__w/firedrake/firedrake/petsc/src/vec/is/utils/isio.c:50
2	[0] PetscViewerHDF5Load() at /__w/firedrake/firedrake/petsc/src/vec/is/utils/hdf5/hdf5io.c:327
2	[0] PetscViewerHDF5Load_Internal() at /__w/firedrake/firedrake/petsc/src/vec/is/utils/hdf5/hdf5io.c:246
2	[0] Unexpected data in file
2	[0] Object (dataset) "order" not stored in group /topologies/channel_topology/dms/firedrake_dm_1_0_0_False_2
2	FAILED firedrake-repo/tests/firedrake/output/test_io_timestepping.py::test_io_timestepping[P2-P1] - petsc4py.PETSc.Error: error code 79
2	[0] DMPlexSectionLoad() at /__w/firedrake/firedrake/petsc/src/dm/impls/plex/plex.c:2660
2	[0] DMPlexSectionLoad_HDF5_Internal() at /__w/firedrake/firedrake/petsc/src/dm/impls/plex/hdf5/plexhdf5.c:2972
2	[0] ISLoad() at /__w/firedrake/firedrake/petsc/src/vec/is/is/interface/index.c:1673
2	[0] ISLoad_Default() at /__w/firedrake/firedrake/petsc/src/vec/is/utils/isio.c:113
2	[0] ISLoad_HDF5() at /__w/firedrake/firedrake/petsc/src/vec/is/utils/isio.c:50
2	[0] PetscViewerHDF5Load() at /__w/firedrake/firedrake/petsc/src/vec/is/utils/hdf5/hdf5io.c:327
2	[0] PetscViewerHDF5Load_Internal() at /__w/firedrake/firedrake/petsc/src/vec/is/utils/hdf5/hdf5io.c:246
2	[0] Unexpected data in file
2	[0] Object (dataset) "order" not stored in group /topologies/channel_topology/dms/firedrake_dm_1_0_0_False_2

@ksagiyam do you know what could be causing this?

[ksagiyam]

We just need ngsPETSc.