Expose num_threads when reading in meshes from XDMFFile and VTKHDF

[jorgensd]

Both are hard-coded to use 1 thread. As these are the main entry points for large meshes this option should be exposed, ref:

dolfinx/cpp/dolfinx/io/XDMFFile.cpp

Line 202 in cc5c6a2

dolfinx::graph::Partitioner{}, mode, max_facet_to_cell_links, 1);

dolfinx/cpp/dolfinx/io/VTKHDF.h

Lines 472 to 475 in cc5c6a2

	return mesh::create_mesh(comm, comm, cells_span, coordinate_elements, comm,
	points_pruned, {(std::size_t)x_shape[0], gdim},
	graph::Partitioner{}, mesh::GhostMode::none,
	max_facet_to_cell_links, 1);