Print the summary of a graph.

        :param str heading: Title of the graph.

def print_summary(self, heading: str) -> None:
        """Print the summary of a graph.

        :param str heading: Title of the graph.
        """
        logger.info(heading)
        logger.info("Number of nodes: {}".format(len(self.graph.vs)))
        logger.info("Number of edges: {}".format(len(self.graph.es)))