add method getLayerGroup

modules/viewer/Viewer.js

@@ -414,6 +414,15 @@ class Viewer {
     return this
   }
 
+  /**
+   *
+   * @param id
+   * @returns {undefined}
+   */
+  getLayerGroup(id) {
+    return this._layerGroupCache[id] || undefined
+  }
+
   /**
    * add a layer
    * @param layer