Change in OntologyTermAdaptor method names:

  fetch_all_by_child_term() --> fetch_all_by_descendant_term()
  fetch_by_child_term()     --> fetch_all_by_child_term()

  fetch_all_by_parent_term()    --> fetch_all_by_ancestor_term()
  fetch_by_parent_term()        --> fetch_all_by_parent_term()


Also, make accessors read-only.