docs: private static void computeCentroid(double[] centroid, Relation<? extends NumberVector> relation, DBIDs ids) {
      Arrays.fill(centroid, 0);
      int dim = centroid.length;
      for(DBIDIter it = ids.iter(); it.valid(); it.advance()) {
        NumberVector v = relation.get(it);
        for(int i = 0; i < dim; i++) {
          centroid[i] += v.doubleValue(i);
        }
      }
      timesEquals(centroid, 1. / ids.size());
    }