Port Turf.hex-grid to java

Make Turf's hex-grid function be available in java.
Hex-grid utility will return a Feature Collection of hexagons or triangles based on a bounding box and the diameter of the cell. This is will enable to do a HeatMap demo in the android-demo-app.

Here is more information about hex-grid:
https://github.com/Turfjs/turf/tree/master/packages/turf-hex-grid

從所引用的 Turf hex-grid 實作開始，並將其行為與 Java SDK 進行比較；該 issue 未指定目標檔案或測試。當 Java 實作能夠根據 bounding box 和單元直徑回傳由六邊形或三角形組成的 feature collection，並支援 android-demo-app 的 heatmap 使用情境時，工作即告完成。