REF: Decide how to implement `_set_ad_order` for pricing objects with manually updated nodes.

[attack68]

    @pytest.mark.skip(reason="no decision on how to use _set_ad_order for manually updated nodes.")
    def test_update_single_key(self):
        # TODO need to decide how _set_or_ad should work with update nodes.

This test exemplifies the issue. What happens if a user supplies their own AD variables - should these be overridden immediately or is their a use case for not doing so,