ENH: two algorithms for the convex hull problem of a set of 2d points on a plain (#1135)

* divide and conquer and brute force algorithms for array-inversions counting

* divide and conquer and brute force algorithms for array-inversions counting

* divide and conquer and brute force algorithms for array-inversions counting

* a naive and divide-and-conquer algorithms for the convex-hull problem

* two convex-hull algorithms, a divide-and-conquer and a naive algorithm

* two convex-hull algorithms, a divide-and-conquer and a naive algorithm

* two convex-hull algorithms, a divide-and-conquer and a naive algorithm