docs: public static <V extends NumberVector> V getPrototypeOrCentroid(Model model, Relation<? extends V> relation, DBIDs ids, NumberVector.Factory<V> factory) {
      assert (ids.size() > 0);
      V v = getPrototype(model, relation, factory);
      return v != null ? v : factory.newNumberVector(Centroid.make(relation, ids));
    }