Add GraphTopologyAddition trait from GraphTopology

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

src/generators.rs

@@ -1,4 +1,4 @@
-use crate::traits::GraphTopology;
+use crate::traits::{GraphTopology, GraphTopologyAddition};
 
 pub fn petersen<G>(
     graph: &mut G,
@@ -7,7 +7,7 @@ pub fn petersen<G>(
     [<G as GraphTopology>::Edge; 15],
 )
 where
-    G: GraphTopology,
+    G: GraphTopologyAddition,
 {
     const N: usize = 5;
     const K: usize = 2;