code: public Feature newCategory(String id, String lemma, List<Span<Term>> references) {
  	idManager.updateCounter(AnnotationType.CATEGORY, id);
  	Feature newCategory = new Feature(id, lemma, references);
  	annotationContainer.add(newCategory, Layer.CATEGORIES, AnnotationType.CATEGORY);
  	return newCategory;
      }
docs: Creates a new category. It receives it's ID as an argument. The category is added to the document.
  @param id the ID of the category.
  @param lemma the lemma of the category.
  @param references different mentions (list of targets) to the same category.
  @return a new coreference.