docs: public double getHullDistance(SphereCluster other) {
  		double distance = 0.0;
  		//get the center through getCenter so subclass have a chance
  		double[] center0 = getCenter();
  		double[] center1 = other.getCenter();
  		distance = distance(center0, center1);
  
  		distance = distance - getRadius() - other.getRadius();
  		return distance;
  	}