clustering-index.go
func NewExpiringClusteringIndex(expiration Minutes) *ClusteringIndex {
	index := &ClusteringIndex{}
	index.streetLevel = NewExpiringPointsIndex(Km(0.5), expiration)
	index.cityLevel = NewExpiringCountIndex(Km(10), expiration)
	index.worldLevel = NewExpiringCountIndex(Km(500), expiration)

	return index
}
func NewExpiringClusteringIndex(expiration Minutes) *ClusteringIndex {
	index := &ClusteringIndex{}
	index.streetLevel = NewExpiringPointsIndex(Km(0.5), expiration)
	index.cityLevel = NewExpiringCountIndex(Km(10), expiration)
	index.worldLevel = NewExpiringCountIndex(Km(500), expiration)

	return index
}
// NewExpiringClusteringIndex creates index that clusters the points at three levels with cell size 0.5, 5 and 500km and
// expires them after expiration minutes.