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

从所引用的 Turf hex-grid 实现开始，并将其行为与 Java SDK 进行比较；该 issue 未指定目标文件或测试。当 Java 实现能够根据 bounding box 和单元直径返回由六边形或三角形组成的 feature collection，并支持 android-demo-app 的 heatmap 用例时，工作即告完成。