code: public int[] getClusterDesignations(int[] designations, int clusters)
      {
          if(merges == null)
              return null;
          return PriorityHAC.assignClusterDesignations(designations, clusters, merges);
      }
docs: Returns the assignment array for that would have been computed for the
  previous data set with the desired number of clusters.
  
  @param designations the array to store the assignments in
  @param clusters the number of clusters desired
  @return the original array passed in, or <tt>null</tt> if no data set has been clustered.
  @see #hasStoredClustering()