optimization of cv::accumulate**
improved cv::ocl::predictOptimalVectorWidth
used new stratehy in cv::accumulate**
changed perf test for ocl_gemm
Update py_setup_in_windows.rst
Merge pull request #2956 from ilya-lavrenov:tapi_accumulate
Merge pull request #3162 from alekcac:fix_in_py_tutor
Merge pull request #3161 from ElenaGvozdeva:ocl_perf_gemm
continue fixing nonfree elements
fixed dependency of stitching module on xfeatures2d module as optional
fixed the initModule_xfeatures2d function that was called in module stitching since it is in another namespace than the standard cv one.
Merge pull request #3163 from StevenPuttemans:fix_nonfree_items