clustering-index.go
func NewClusteringIndex() *ClusteringIndex {
	index := &ClusteringIndex{}
	index.streetLevel = NewPointsIndex(Km(0.5))
	index.cityLevel = NewCountIndex(Km(10))
	index.worldLevel = NewCountIndex(Km(500))

	return index
}
func NewClusteringIndex() *ClusteringIndex {
	index := &ClusteringIndex{}
	index.streetLevel = NewPointsIndex(Km(0.5))
	index.cityLevel = NewCountIndex(Km(10))
	index.worldLevel = NewCountIndex(Km(500))

	return index
}
// NewClusteringIndex creates index that clusters the points at three levels with cell size 0.5, 5 and 500km.
// Useful for creating maps.