Find all the nodes where there is an edge from the specified node to that node.
        Returns a list of node ids.

def neighbors(self, node_id):
        """Find all the nodes where there is an edge from the specified node to that node.
        Returns a list of node ids."""
        node = self.get_node(node_id)
        flattened_nodes_list = []
        for a, b in [self.get_edge(edge_id)['vertices'] for edge_id in node['edges']]:
            flattened_nodes_list.append(a)
            flattened_nodes_list.append(b)
        node_set = set(flattened_nodes_list)
        if node_id in node_set:
            node_set.remove(node_id)
        return [nid for nid in node_set]