code: public List<Triple<F,L,Double>> getTopFeatures(double threshold, boolean useMagnitude, int numFeatures)
    {
      return getTopFeatures(null, threshold, useMagnitude, numFeatures, true);
    }
docs: Returns list of top features with weight above a certain threshold
  (list is descending and across all labels)
  @param threshold  Threshold above which we will count the feature
  @param useMagnitude Whether the notion of "large" should ignore
  the sign of the feature weight.
  @param numFeatures  How many top features to return (-1 for unlimited)
  @return List of triples indicating feature, label, weight