code: public static <KEYTYPE, DATATYPE, ITEMTYPE extends ITreeItemWithID <KEYTYPE, DATATYPE, ITEMTYPE>> void sortByValue (@Nonnull final IBasicTree <DATATYPE, ITEMTYPE> aTree,
                                                                                                                        @Nonnull final Comparator <? super DATATYPE> aValueComparator)
    {
      _sort (aTree, Comparator.comparing (IBasicTreeItem::getData, aValueComparator));
    }
docs: Sort each level of the passed tree on the value with the specified
  comparator.
  
  @param <KEYTYPE>
  Tree item key type
  @param <DATATYPE>
  Tree item data type
  @param <ITEMTYPE>
  Tree item type
  @param aTree
  The tree to be sorted.
  @param aValueComparator
  The comparator to be used for sorting the tree item keys on each
  level.