Add todo that was accidentally removed

src/models/graph.rs

@@ -116,6 +116,7 @@ impl Graph {
     }
 
     fn step_incidence(&self, incidence: &mut Option<IncidenceSlot>) -> Option<(VertexSlot, Edge)> {
+        // TODO: Benchmark storing full Index (one read, larger entries) vs. slot + get_idx() (two reads, smaller entries).
         let current = (*incidence)?;
         let e = self.incidences.get_idx(current.0).unwrap();
         let entry = self.incidences[e];