mixed element

[pierricmora]

There is a RuntimeError when calling create_interpolation_matrix(V, ...) with mixed elements such as created from basix.ufl.mixed_element([elt1, elt2]). This simple change seems to extend (pytest passes) to these cases:

• in interpolation_utils.py, line 42:

try:
    value_size = np.prod(V.element.basix_element.value_shape)
except RuntimeError:  # No Basix element available. Maybe this is a mixed element?
    value_size = np.prod(V.value_shape)

• in interpolation.py, line 111:

try:
    second_dimension = max(V.dofmap.bs, np.prod(V.element.basix_element.value_shape))
except RuntimeError:  # No Basix element available. Maybe this is a mixed element?
    second_dimension = max(V.dofmap.bs, np.prod(V.value_shape))

Would you agree to add or consider a PR?