Returns a reference to the dict of target node id to (edge_id, edge)

def edge_by_target(self):
        """Returns a reference to the dict of target node id to (edge_id, edge)"""
        if self._edge_by_target is None:
            self._edge_by_target = reverse_edge_by_source_dict(self._edge_by_source_id,
                                                               self._nexson_tree['^ot:rootNodeId'])
        return self._edge_by_target