more work on getting model selection working.
sort by longest strings first, so that iterator() & _get_sub_obj_recurse() don't break early, leaving a child where later on they could encounter a grandchild.
raise an error if the Model class given is wrong.
more tests
splitting tests further, renaming them.
test_mixing_strings_and_classes_with_children and test_selecting_all_subclasses_specifically_children fail.
seeing if order makes a difference.
These should be children, because grandchildren1/1_2 are both subclasses of this child type.
last failing test under 1.6