Updated todos

src/models/frozen_graph.rs

@@ -96,6 +96,7 @@ pub struct FrozenGraph {
 }
 
 impl FrozenGraph {
+    // TODO: Move this to a new FromGraph trait and adapt the MakeTestGraph impl to use helper functions like "fn fixture_empty<G: FromGraph>() -> G { G::from_graph(&AppendGraph::empty()).0 }".
     /// Creates a graph instance as a copy of another graph.
     ///
     /// Returns the copied graph instance and two mappings of the `source` graph vertices and edges