Add an outward edge to a vertex

        :param fr: The source vertex.
        :param to: The name of the outward edge.

def add_edge(self, fr, to):
        """ Add an outward edge to a vertex

        :param fr: The source vertex.
        :param to: The name of the outward edge.
        """
        if fr not in set(self.graph.vs):  # ToDo: find out why item can be in set but not dict
            raise ValueError('can not connect unknown vertices in n-partite graphs, {!r} missing'.format(fr))
        elif to not in set(self.graph.vs):
            raise ValueError('can not connect unknown vertices in n-partite graphs, {!r} missing'.format(to))
        self.graph.add_edge(fr, to)