Introduce a regulator trait

This will provide a common interface for Lorentz product regulators,
curvature regulators, and hopefully all the other regulators too.

app-proto/src/add_remove.rs

@@ -199,7 +199,7 @@ pub fn AddRemove() -> View {
                             .try_into()
                             .unwrap()
                     );
-                    state.assembly.insert_new_regulator(subjects);
+                    state.assembly.insert_new_product_regulator(subjects);
                     state.selection.update(|sel| sel.clear());
                 }
             ) { "🔗" }