Correct cellname for point/vertex type for UFL (#851)

* Expose subentity types to python for easy creation of elements on entites.

* Mypy fixes

* Add vertex submesh

python/basix/ufl.py

@@ -103,6 +103,8 @@ def __init__(
     ):
         """Initialise the element."""
         self._repr = repr
+        if cellname == "point":
+            cellname = "vertex"
         self._cellname = cellname
         self._reference_value_shape = reference_value_shape
         self._degree = degree