Add example with Kuwahara filter

[martin-carrasco]

Description

Added an implementation of the Kuwahara adaptive filter working for grayscale images

This is part of my competency test solution for GSoC 2020. I would appreciate any feedback or comments.

References

https://d-nb.info/1094419265/34

Further Development

• Implement an channel agnostic version and add to filter.hpp
• Document

Tasklist

• Add test case(s)
• Ensure all CI builds pass
• Review and approve

[mloskot]

Thanks for the example. For the competency test it looks good.
For submission to GIL as example, it needs copyright notice, Jamfile/CMakeLists.txt update, some formatting (even minimal using the .clang-format), according to GIL guidelines. However, it will not be accepted before the GSoC students selection is completed.