skm_rgs_cpp: skm_rgs_cpp
In gyang274/skm: Selective k-Means

solve skm with single and random size k s_init stratified sampled w.r.t g

1	skm_rgs_cpp(x, k, g, s_must, max_it)

`x`	an m x n matrix often m < n, as a convention index rows of x with s, and cols of x with t so x(i, j) can be expressed as (s_i, t_j) equally.
`k`	number of index to be selected from x row index start from 0.
`g`	stratify structure, often info on grouping of v so that algorithm should make random initialization from stratified sample across groups.
`s_must`	an index vector set should be selected before selecting other index.
`max_it`	max number of iterations can run for optimizing result. max number of iterations within a single initial run on optimal path.