Fix SelectionUpdateOp update always repainting (#262)

Override the equalValue member to do a deep equality test
The default equality test doesn't actually look at values.
This causes a render after every selection, even if nothing has changed.
For graphs with a lot of points this caused a severe slowdown.

lib/src/interaction/selection/selection.dart

@@ -415,4 +415,12 @@ class SelectionUpdateOp extends Operator<AttributesGroups> {
     }
     return rst;
   }
+
+  ///Need to provide a semantic equality comparison since
+  ///AttributesGroups are really just an alias to a list
+  @override
+  bool equalValue(AttributesGroups a, AttributesGroups b){
+    return deepCollectionEquals(a, b);
+  }
+
 }