Get the pathways associated with an edge.

        Parameters
        -----------
        edge_id : tup(int, int)

        Returns
        -----------
        tup(str, str)|None, the edge as a pair of 2 pathways if the edge id
          is in this network

def get_edge_pathways(self, edge_id):
        """Get the pathways associated with an edge.

        Parameters
        -----------
        edge_id : tup(int, int)

        Returns
        -----------
        tup(str, str)|None, the edge as a pair of 2 pathways if the edge id
          is in this network
        """
        vertex0_id, vertex1_id = edge_id
        pw0 = self.get_pathway_from_vertex_id(vertex0_id)
        pw1 = self.get_pathway_from_vertex_id(vertex1_id)
        if not pw0 or not pw1:
            return None
        return (pw0, pw1)