Merge pull request #18857 from OrestChura:oc/kmeans

Merge pull request #18857 from OrestChura:oc/kmeans

[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)