SVM sigmoid kernel fix (issue #13621) (#13718)

* Added test for sigmoid case for retrieving support vectors

* undo unhelpful test

* add test for sigmoid SVM with data that is easily separable into two concentric circles

* Update sigmoid kernel to use tanh(gamma * <x, y> + coef0) instead of -tanh(gamma * <x, y> + coef0)

* remove unnecessary constraint on coef0

* cleanup

* fixing inappropriate use of doubles

* Add f to float literal

* replace CV_Assert with ASSERT_EQ where appropriate

* spatialGradient() reworked to use wide universal intrinsics

* Moved row pointers inside loops

* Added DMatch bindings

* Added CR which was accidently removed to maintain consistency

* Removed trailing white-space

* imgproc: cleanup medianBlur_8u_O1 code

Unnecessary per-channel buffers: H[c] / lut[c]

* imgproc(medianBlur_8u_O1): use CV_SIMD_WIDTH for alignment

PyrDown: Fix bug #12961 (#13672)

* Force unaligned pointer and create test

* More cross-platform solution

* MSVC expects a proper order

* Remove useless clang macro

* Return vector of MetaArg instead of MatDesc and fix test with ADL check

* Fix construction of vector

* Change names and tests according to review
Also add GAPI_EXPORTS prefix for two ostream operators

* Add note about building for pkg-config

* minor fixes

* Add version of descr_of function taking vector of Mat

* Overload descr_of function for cv::Mat

* Add template function instead of copypasting and change tests a little

* __shfl_up_sync with proper mask value for CUDA >= 9

* BlockScanInclusive for CUDA >= 9

* compatible_shfl_up for use in integral.hpp

* Use CLAHE in cudev

* Add tests for BlockScan