* trying to fix the custom AVX2 builder test failures (false alarms)

* fixed compile error with CPU_BASELINE=AVX2 on x86; raised tolerance thresholds in a couple of tests

* fixed compile error with CPU_BASELINE=AVX2 on x86; raised tolerance thresholds in a couple of tests

* fixed compile error with CPU_BASELINE=AVX2 on x86; raised tolerance thresholds in a couple of tests

* seemingly disabled false alarm warning in surf.cpp; increased tolerance thresholds in the tests for SolvePnP and in DNN/ENet

- allow 5% of changed pixels with intensity difference <= 1

Refactor decolor (#11035)

* decolor: modernize deprecated headers (math.h -> cmath)

* refactor contrast_preserve.cpp

* refactor contrast_preserve.hpp (add static/const)

* refactor contrast_preserve.hpp (join assignment and declaration)

* refactor contrast_preserve.hpp (format)

* refactor test_decolor.cpp (indent)

* refactor contrast_preserve.hpp (const)

* contrast_preserve.hpp : optimize by minMaxLoc

* fix trailing-whitespace

* fix warning C4267 on VC++ x64
(conversion from 'size_t' to 'int', possible loss of data)

* use cvRound instead of round_num

* decrease indentation of test_decolor.cpp

* remove pow() to optimize

* remove redundant Mat initialization

- removed tr1 usage (dropped in C++17)
- moved includes of vector/map/iostream/limits into ts.hpp
- require opencv_test + anonymous namespace (added compile check)
- fixed norm() usage (must be from cvtest::norm for checks) and other conflict functions
- added missing license headers

  * make test more reproducible

RGB2Lab_f and trilinear interpolation code are in separate branch; cubeRoot(x) instead of std::pow(x, 1.f/3.f)

file with internal accuracy&speed tests moved to lab_tetra branch

* MulSpectrums
* StereoConstantSpaceBP
* BruteForceNonLocalMeans

significantly improved parallel non-local means by using granularity parameter in parallel_for_ loop. Because the algorithm deals with sliding sums, it's essential that each thread has enough work to do, otherwise the algorithm gets higher theoretical complexity and thus there is no speedup comparing to 1-thread code (at best).

trying to solve compile problems; temporarily disabled some tests for just added optimization algo's

removed optim module; moved its functionality to core and photo modules; moved drawing functions from core to imgproc. Removed FilterEngine etc. from public API