Add attribute to the given node.

        @type  node: node
        @param node: Node identifier

        @type  attr: tuple
        @param attr: Node attribute specified as a tuple in the form (attribute, value).

def add_node_attribute(self, node, attr):
        """
        Add attribute to the given node.

        @type  node: node
        @param node: Node identifier

        @type  attr: tuple
        @param attr: Node attribute specified as a tuple in the form (attribute, value).
        """
        self.node_attr[node] = self.node_attr[node] + [attr]