geo-index.go
func newGeoIndex(resolution Meters, newEntry func() interface{}) *geoIndex {
	return &geoIndex{resolution, make(map[cell]interface{}), newEntry}
}
func newGeoIndex(resolution Meters, newEntry func() interface{}) *geoIndex {
	return &geoIndex{resolution, make(map[cell]interface{}), newEntry}
}
// Creates new geo index with resolution a function that returns a new entry that is stored in each cell.