Keep the geometrical dimension of parent mesh when refining

[jorgensd]

Bug reported at discourse
MWE:

import dolfinx
import ufl
import numpy as np
points = np.array([[0, 0],  [1.2, 0],  [1.5, 1],  [0.2, 1]])
cells = np.array([[0, 1, 2],  [0, 2, 3]])
cell = ufl.Cell("triangle", geometric_dimension=2)
domain = ufl.Mesh(ufl.VectorElement("Lagrange", cell, 1))
mesh = dolfinx.mesh.create_mesh(MPI.COMM_WORLD, cells, points, domain)
print(mesh.topology.dim, mesh.geometry.dim)  # 2   2

mesh2=refine(mesh)
print(mesh2.topology.dim, mesh2.geometry.dim)  # 2 3

This is the least intrusive way of slicing the new_vertex_coordinates, as the returned midpoints are 3D.