Oasis - Build spatial index for charge stations based on Google::S2

[CodeBear801]

Subtask of #231, implements #236

• It loads points data from previous step and build spatial index to answer nearbyQuery
• It requires to record pre-processing data to files, so next time just need load data back to re-build spatial information

[CodeBear801]

Code saving:

Print Cells in sorted distance

func cellIDFromString(s string) s2.CellID {
	level := len(s) - 2
	if level < 0 || level > 30 {
		return s2.CellID(0)
	}
	face := int(s[0] - '0')
	if face < 0 || face > 5 || s[1] != '/' {
		return s2.CellID(0)
	}
	id := s2.CellIDFromFace(face)
	for i := 2; i < len(s); i++ {
		childPos := s[i] - '0'
		if childPos < 0 || childPos > 3 {
			return s2.CellID(0)
		}
		id = id.Children()[childPos]
	}
	return id
}

	keys := make([]string, 0, len(cellID2PointIDs))
	for k := range cellID2PointIDs {
		keys = append(keys, k.String())
	}
	sort.Strings(keys)
	for _, kStr := range keys {
		k := cellIDFromString(kStr)
		fmt.Printf("%d : {", k)
		for i, p := range cellID2PointIDs[k] {
			if i != 0 {
				fmt.Printf(",")
			}
			fmt.Printf("%d", p)
		}
		fmt.Printf("}, // %v \n", k)
	}