Adds a new, blank node to the graph.
        Returns the node id of the new node.

def new_node(self):
        """Adds a new, blank node to the graph.
        Returns the node id of the new node."""
        node_id = self.generate_node_id()

        node = {'id': node_id,
                'edges': [],
                'data': {}
        }

        self.nodes[node_id] = node

        self._num_nodes += 1

        return node_id