graph: change Edge interface to include ID method

This is to allow future handling of multigraphs.

graph/topo/clique_graph.go

@@ -101,6 +101,9 @@ func (e CliqueGraphEdge) From() graph.Node { return e.from }
 // To returns the to node of the edge.
 func (e CliqueGraphEdge) To() graph.Node { return e.to }
 
+// ID returns zero.
+func (e CliqueGraphEdge) ID() int64 { return 0 }
+
 // Nodes returns the common nodes in the cliques of the underlying graph
 // corresponding to the from and to nodes in the clique graph.
 func (e CliqueGraphEdge) Nodes() []graph.Node { return e.nodes }