Add attribute to resolve activity by label.

eraser-base/src/main/jastadd/Navigation.jrag

@@ -108,6 +108,14 @@ aspect Navigation {
     }
     return java.util.Optional.empty();
   }
+  syn java.util.Optional<Activity> Root.resolveActivity(String label) {
+    for (Activity activity : getMachineLearningRoot().getActivityList()) {
+      if (activity.getLabel().equals(label)) {
+        return java.util.Optional.of(activity);
+      }
+    }
+    return java.util.Optional.empty();
+  }
 
   //--- resolveChangeEvent ---
   syn java.util.Optional<ChangeEvent> Root.resolveChangeEvent(int identifier) {