Add Default implementation for AppendGraph

src/models/append_graph.rs

@@ -61,6 +61,12 @@ impl AppendGraph {
     }
 }
 
+impl Default for AppendGraph {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl GraphTopology for AppendGraph {
     type Vertex = Vertex;