made GPU Feature Extractor download/convertKeypoints() methods static (Feature #2007)
made GPU Feature Extractor download/convertKeypoints() methods static (Feature #2007)

apavlenko/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv