Add an outward edge to a vertex

        :param v: The source vertex.
        :param e: The name of the outward edge.

def add_outward_edge(self, v, e):
        """ Add an outward edge to a vertex

        :param v: The source vertex.
        :param e: The name of the outward edge.
        """
        self.add_vertex(v)
        self.graph.add_vertex(e)
        self.es.add(e)
        self.graph.add_edge(e, v)