count-index.go
func NewCountIndex(resolution Meters) *CountIndex {
	newCounter := func() interface{} {
		return &singleValueAccumulatingCounter{}
	}

	return &CountIndex{newGeoIndex(resolution, newCounter), make(map[string]Point)}
}
func NewCountIndex(resolution Meters) *CountIndex {
	newCounter := func() interface{} {
		return &singleValueAccumulatingCounter{}
	}

	return &CountIndex{newGeoIndex(resolution, newCounter), make(map[string]Point)}
}
// NewCountIndex creates an index which counts the points in each cell.