code: func (csd CentroidSquaredDistance) ClusterDistance(c1, c2 *Cluster) float64 {
  	// save the function call, is this faster?
  	d0 := (c1.Centroid[0] - c2.Centroid[0])
  	d1 := (c1.Centroid[1] - c2.Centroid[1])
  	return d0*d0 + d1*d1
  }
docs: // ClusterDistance computes the squared euclidean distance between the cluster centroids.