code: function forEachItemGroup (callback) {
          this.items.each(function (item) {
              if (ItemGroup.isItemGroup(item)) {
                  item.forEachItemGroup(callback);
                  callback(item, this); // eslint-disable-line callback-return
              }
          }, this);
      }
docs: Calls the callback for each itemgroup belonging to itself. All ItemGroups encountered will also,
  call the callback on their own ItemGroups
  
  @private
  @param {Function} callback