docs: public static Centroid make(Relation<? extends NumberVector> relation, DBIDs ids) {
      final int dim = RelationUtil.dimensionality(relation);
      Centroid c = new Centroid(dim);
      double[] elems = c.elements;
      int count = 0;
      for(DBIDIter iter = ids.iter(); iter.valid(); iter.advance()) {
        NumberVector v = relation.get(iter);
        for(int i = 0; i < dim; i++) {
          elems[i] += v.doubleValue(i);
        }
        count += 1;
      }
      if(count == 0) {
        return c;
      }
      for(int i = 0; i < dim; i++) {
        elems[i] /= count;
      }
      c.wsum = count;
      return c;
    }