code: func (ps PointSet) Centroid() *Point {
  	x := 0.0
  	y := 0.0
  	numPoints := float64(len(ps))
  	for _, point := range ps {
  		x += point[0]
  		y += point[1]
  	}
  	return &Point{x / numPoints, y / numPoints}
  }
docs: // Centroid returns the average x and y coordinate of the point set.
  // This can also be used for small clusters of lat/lng points.