code: public List<List<DataPoint>> getClusterDesignations(int clusters)
      {
          if(!hasStoredClustering())
              return null;
          int[] assignments = new int[curDataSet.size()];
          return createClusterListFromAssignmentArray(assignments, curDataSet);
      }
docs: Returns the assignment array for that would have been computed for the
  previous data set with the desired number of clusters.
  
  @param clusters the number of clusters desired
  @return the list of data points in each cluster, or <tt>null</tt> if no
  data set has been clustered.
  @see #hasStoredClustering()