Enhance TopologicalOrderIterator to support BOTTOM_UP

case BOTTOM_UP:
         // TODO jvs 4-Apr-2006:  enhance TopologicalOrderIterator
      // to support reverse walk.
      final List<HepRelVertex> list = new ArrayList<>();
      for (HepRelVertex vertex : TopologicalOrderIterator.of(graph)) {
        list.add(vertex);
      }
      Collections.reverse(list);
      return list.iterator();