Support VertexOnlyMesh in 1D mesh

[ReubenHill]

At present creating a VertexOnlyMesh within an interval mesh causes a NotImplementedError. If this is removed then,

with test.py containing

from firedrake import *
m = UnitIntervalMesh(1)
vertices = [(0.,), (0.1,), (0.9,), (1.0,)]
vm = VertexOnlyMesh(m, vertices)

I get

Traceback (most recent call last):
  File "test.py", line 4, in <module>
    vm = VertexOnlyMesh(m, vertices)
  File "/Users/rwh10/firedrake/src/firedrake/firedrake/mesh.py", line 1733, in VertexOnlyMesh
    swarm = _pic_swarm_in_plex(mesh.topology._plex, vertexcoords, fields=[("parentcellnum", 1, IntType)])
  File "/Users/rwh10/firedrake/src/firedrake/firedrake/mesh.py", line 1846, in _pic_swarm_in_plex
    swarm.setPointCoordinates(coords, redundant=False, mode=PETSc.InsertMode.INSERT_VALUES)
  File "PETSc/DMSwarm.pyx", line 200, in petsc4py.PETSc.DMSwarm.setPointCoordinates
petsc4py.PETSc.Error: error code 63
[0] DMSwarmSetPointCoordinates() line 305 in /Users/rwh10/firedrake/src/petsc/src/dm/impls/swarm/swarmpic.c
[0] DMLocatePoints() line 6550 in /Users/rwh10/firedrake/src/petsc/src/dm/interface/dm.c
[0] DMLocatePoints_Plex() line 738 in /Users/rwh10/firedrake/src/petsc/src/dm/impls/plex/plexgeometry.c
[0] DMPlexLocatePoint_Internal() line 460 in /Users/rwh10/firedrake/src/petsc/src/dm/impls/plex/plexgeometry.c
[0] Argument out of range
[0] No point location for cell 0 with type segment

Originally posted by @ReubenHill in #1638 (comment)

Suggested solution:

OK, so if you wanted to support this you could implement DMPlexLocatePoint_Simplex_1D_Internal and hook it up in DMPlexLocatePoint_Internal

Originally posted by @wence- in #1638 (comment)

[ReubenHill]

Resolved by #2175