TurfMeasurements should have a function for determining bearing deltas

TurfMeasurement.bearing(Point point1, Point point2

This function can return -180.0, 180.0.

In order to calculate the difference between two bearings. Downstream needs to create a normalizing function which can determine that the difference between -179.0 and 179.0, is 2 degrees.

double TurfMeasurements.shortestBearingDiff(double bearing1, double bearing2) {
  ...
}

This function should be proven with unit tests.

首先阅读 services-turf/src/main/java/com/mapbox/turf/TurfMeasurement.java，尤其是 issue 中引用的 bearing 函数。添加所请求的 bearing-difference 函数和单元测试，覆盖 -179.0 和 179.0 的情况，包括可能得到 -180.0 和 180.0 的结果；完成的标准是测试能够证明所得差值是最短差值。