Core API: loadChildTopics() returns object (#699).

The `loadChildTopics()` methods return the object itself, thus allowing call chaining (formerly `void`).

{{{
DeepaMehtaObject
    DeepaMehtaObject loadChildTopics();
    DeepaMehtaObject loadChildTopics(String childTypeUri);

Topic
    Topic loadChildTopics();
    Topic loadChildTopics(String childTypeUri);

Association
    Association loadChildTopics();
    Association loadChildTopics(String childTypeUri);
}}}

See #699.