[G-API]: kmeans() Standard Kernel Implementation
* cv::gapi::kmeans kernel implementation
- 4 overloads:
- standard GMat - for any dimensionality
- GMat without bestLabels initialization
- GArray<Point2f> - for 2D
- GArray<Point3f> - for 3D
- Accuracy tests:
- for every input - 2 tests
1) without initializing. In this case, no comparison with cv::kmeans is done as kmeans uses random auto-initialization
2) with initialization
- in both cases, only 1 attempt is done as after first attempt kmeans initializes bestLabels randomly
* Addressing comments
- bestLabels is returned to its original place among parameters
- checkVector and isPointsVector functions are merged into one, shared between core.hpp & imgproc.hpp by placing it into gmat.hpp (and implementation - to gmat.cpp)
- typos corrected
* addressing comments
- unified names in tests
- const added
- typos
* Addressing comments
- fixed the doc note
- ddepth -> expectedDepth, `< 0 ` -> `== -1`
* Fix unsupported cases of input Mat
- supported: multiple channels, reversed width
- added test cases for those
- added notes in docs
- refactored checkVector to return dimentionality along with quantity
* Addressing comments
- makes chackVector smaller and (maybe) clearer
* Addressing comments
* Addressing comments
- cv::checkVector -> cv::gapi::detail
* Addressing comments
- Changed checkVector: returns bool, quantity & dimensionality as references
* Addressing comments
- Polishing checkVector
- FIXME added
* Addressing discussion
- checkVector: added overload, separate two different functionalities
- depth assert - out of the function
* Addressing comments
- quantity -> amount, dimensionality -> dim
- Fix typos
* Addressing comments
- fix docs
- use 2 variable's definitions instead of one (for all non-trivial variables)
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