Add Incidence struct for GraphTopology to use

src/models.rs

@@ -5,5 +5,7 @@ pub mod graph;
 
 // TODO: Compressed-sparse-row graph model.
 
-pub use append_graph::{AppendGraph, AppendGraphEdgeMap, AppendGraphVertexMap};
-pub use graph::{Graph, GraphEdgeMap, GraphVertexMap};
+pub use append_graph::{
+    AppendGraph, AppendGraphEdgeMap, AppendGraphIncidence, AppendGraphVertexMap,
+};
+pub use graph::{Graph, GraphEdgeMap, GraphIncidence, GraphVertexMap};