code: public boolean addFeatureVector(DBIDRef id, NumberVector vector) {
      if(contains(vector)) {
        ids.add(id);
        return true;
      }
      return false;
    }
docs: Adds the id of the specified feature vector to this unit, if this unit
  contains the feature vector.
  
  @param id Vector id
  @param vector the feature vector to be added
  @return true, if this unit contains the specified feature vector, false
  otherwise