Remove a node from the graph.
        
        @type  node: node
        @param node: Node identifier.

def del_node(self, node):
        """
        Remove a node from the graph.
        
        @type  node: node
        @param node: Node identifier.
        """
        for each in list(self.neighbors(node)):
            if (each != node):
                self.del_edge((each, node))
        del(self.node_neighbors[node])
        del(self.node_attr[node])