clustering-index.go
func (index *ClusteringIndex) Add(point Point) {
	index.streetLevel.Add(point)
	index.cityLevel.Add(point)
	index.worldLevel.Add(point)
}
func (index *ClusteringIndex) Add(point Point) {
	index.streetLevel.Add(point)
	index.cityLevel.Add(point)
	index.worldLevel.Add(point)
}
// Add adds a point.