Add GraphTopologyAddition trait from GraphTopology

Move capacity and add methods from GraphTopology trait to new GraphTopologyAddition trait

src/lib.rs

@@ -136,7 +136,9 @@ pub mod traits;
 
 /// Convenience re-exports of graph topology traits for common use.
 pub mod prelude {
-    pub use crate::traits::{GraphTopology, GraphTopologyDeletion, Incidence, IncidenceCursor};
+    pub use crate::traits::{
+        GraphTopology, GraphTopologyAddition, GraphTopologyDeletion, Incidence, IncidenceCursor,
+    };
 }
 
 mod testing;