register dependencies between trees

def _register_dependencies(self):
        """ register dependencies between trees"""
        for tree_name, context_entry in context.timetable_context.items():
            tree = self.trees[tree_name]
            assert isinstance(tree, MultiLevelTree)
            for dependent_on in context_entry.dependent_on:
                dependent_on_tree = self.trees[dependent_on]
                assert isinstance(dependent_on_tree, MultiLevelTree)
                tree.register_dependent_on(dependent_on_tree)