code: public List<double[]> addToClusteringCenters(List<double[]> clustering) {
  		if (center != null && getClusteringFeature() != null) {
  			clustering.add(getClusteringFeature().toClusterCenter());
  		}
  		for (ClusteringTreeNode child : children) {
  			child.addToClusteringCenters(clustering);
  		}
  		return clustering;
  	}
docs: Adds all clustering centers of the ClusterFeatures of the tree with this
  node as the root to a List of points.
  
  @param clustering
  the List to add the clustering centers too.
  @return the input List