* Add HPX backend for OpenCV implementation
Adds hpx backend for cv::parallel_for_() calls respecting the nstripes chunking parameter. C++ code for the backend is added to modules/core/parallel.cpp. Also, the necessary changes to cmake files are introduced.
Backend can operate in 2 versions (selectable by cmake build option WITH_HPX_STARTSTOP): hpx (runtime always on) and hpx_startstop (start and stop the backend for each cv::parallel_for_() call)

* WIP: Conditionally include hpx_main.hpp to tests in core module
Header hpx_main.hpp is included to both core/perf/perf_main.cpp and core/test/test_main.cpp.
The changes to cmake files for linking hpx library to above mentioned test executalbles are proposed but have issues.

* Add coditional iclusion of hpx_main.hpp to cpp cpu modules

* Remove start/stop version of hpx backend

except header file with conversion routines (will be removed in OpenCV 4.0)

Found via `codespell -q 3 -I ../opencv-whitelist.txt --skip="./3rdparty"`

it is reusing CAP_INTELPERC* enums as Intel PerC is deprecated and
librealsense replaces it.

  * remove NVidia and Nvidia
  * replace Cuda with CUDA
  * keep the letters for API

to reduce overhead of gradle builds

Found via `codespell -i 3 -w --skip="./3rdparty" -I ../opencv-whitelist.txt`

fix missing "opencv2/core/opencl" headers from core module (updated install list)

Introduce CMake variable OPENCV_MAP_IMPORTED_CONFIG which controls this behaviour
Added defaults for MSVS targets

- Optimizations set change. Now IPP integrations will provide code for SSE42, AVX2 and AVX512 (SKX) CPUs only. For HW below SSE42 IPP code is disabled.
- Performance regressions fixes for IPP code paths;
- cv::boxFilter integration improvement;
- cv::filter2D integration improvement;

This is VS2017 Preview. It makes sure the version is detected properly and the INSTALL target then correctly installs to x64/vc15 (the same as MSVC1910).

general:
- all iterative tests have been replaced with parameterized tests
- old-style try..catch tests have been modified to use EXPECT_/ASSERT_ gtest macros
- added temporary files cleanup
- modified MatComparator error message formatting

imgcodecs:
- test_grfmt.cpp split to test_jpg.cpp, test_png.cpp, test_tiff.cpp, etc.

videoio:
- added public HAVE_VIDEO_INPUT, HAVE_VIDEO_OUTPUT definitions to cvconfig.h
- built-in MotionJPEG codec could not be tested on some platforms (read_write test was disabled if ffmpeg is off, encoding/decoding was handled by ffmpeg otherwise).
- image-related tests moved to imgcodecs (Videoio_Image)
- several property get/set tests have been combined into one
- added MotionJPEG test video to opencv_extra

- cmake dependencies search (WITH_MFX option)
- raw H264, H265, MPEG2 encoding and decoding
- tests for supported formats

- eliminate CMake 3.5+ warnings
- removed usage of dropped "OPENCV_CAMERA_MODULES" variable
- partial support for Android NDK 11+

* moved BLAS/LAPACK detection scripts from opencv_contrib/dnn to the main repository.

* trying to fix the bug with undefined symbols sgesdd_ and dgesdd_

* removed extra whitespaces; disabled LAPACK on IOS

find_package allows to specify optional components. This way, the
command will not fail if any of the components marked as optional was
not found. This is useful in cases where components such as xfeatures2d,
viz etc. are not available either because they were not compiled by the
user or package maintainers decided to not provide the packages at all.

The user can check the availability of the optional component using the
OpenCV_<component>_FOUND variable.