API: assure_object_is_in_list: this function does now check whether an object (not an entity) is in the list

Added some utitlity functions and corresponding tests:
assure_entity_is_in_list for checking whether an entity has another
entity in one of its properties and possibly sets it
assure_has_parent checks whether an entity has a parent and possibly
adds it
insert_id_based_on_name and get_ids_for_entities_with_names for adding
ids to entities in a container such that they can be updated