Fix extract_subelement_component

[pbrubeck]

No description provided.

[angus-g]

I think this still needs the tweak to value_shape (I don't really understand how value_shape(gdim) and reference_value_shape differ, other than the former no longer existing):

  File "/home/firedrake/g-adopt/gadopt/utility.py", line 163, in is_continuous
    sub_element_index, _ = elem.extract_subelement_component(mesh, int(expr.ufl_operands[1][0]))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/firedrake/firedrake/src/fiat/finat/ufl/mixedelement.py", line 145, in extract_subelement_component
    self._check_component(domain, i)
  File "/home/firedrake/firedrake/src/fiat/finat/ufl/finiteelementbase.py", line 146, in _check_component
    sh = self.value_shape(domain.geometric_dimension())
         ^^^^^^^^^^^^^^^^
AttributeError: 'MixedElement' object has no attribute 'value_shape'