core: kmeans refactoring
core: kmeans refactoring

- reduce scope of i,k,j variables
- use cv::AutoBuffer
- template<bool onlyDistance> class KMeansDistanceComputer
- eliminate manual unrolling: CV_ENABLE_UNROLLED

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv