code: public LinearClassifier<L, F> trainClassifier(Reference<Collection<Datum<L, F>>> ref) {
      Collection<Datum<L, F>> examples = ref.get();
      return trainClassifier(examples);
    }
docs: Takes a {@link Reference} to a {@link Collection} of {@link Datum}
  objects and gives you back a {@link Classifier} trained on them
  
  @param ref {@link Reference} to a {@link Collection} of {@link
  Datum} objects to train the classifier on
  @return A Classifier trained on a collection of Datum