Expose JIT MPI communicator through to form

[jhale]

Allows the compilation of forms on a communicator different to the mesh. The hypothesis is that this may be useful for setting up JIT cache on HPC runs across multiple nodes with shared memory-backed /tmp - this is still to test.

Example of use:

from mpi4py import MPI
from dolfinx.mesh import create_unit_square
from dolfinx.fem import functionspace, form
import ufl

mesh = create_unit_square(MPI.COMM_WORLD, 10, 10)
V = functionspace(mesh, ("CG", 1))
u = ufl.TrialFunction(V)
v = ufl.TestFunction(V)
a = ufl.inner(u, v)*ufl.dx

# See https://docs.open-mpi.org/en/v5.0.x/man-openmpi/man3/MPI_Comm_split_type.3.html#split-types
shared_mem_comm = MPI.COMM_WORLD.Split_type(MPI.COMM_TYPE_SHARED, key=MPI.COMM_WORLD.rank)
a_dolfinx = form(a, jit_comm=shared_mem_comm)

The default behaviour (use the communicator from the form's mesh) is retained when jit_comm=None, which is also the default argument.

[jorgensd]

I think we should do it as a one-liner. Otherwise good