code: public static void quickSelect(ArrayModifiableDBIDs data, Comparator<? super DBIDRef> comparator, int rank) {
      quickSelect(data, comparator, 0, data.size(), rank);
    }
docs: QuickSelect is essentially quicksort, except that we only "sort" that half
  of the array that we are interested in.
  
  Note: the array is <b>modified</b> by this.
  
  @param data Data to process
  @param comparator Comparator to use
  @param rank Rank position that we are interested in (integer!)