Fix calculus for likeness of colors

[starturtle]

Right now, colors are computed as points in a 3D HSV cone space, and their likeness is computed as the distance between two of those points.
However, as soon as all of them have full saturation, the result is very, very poor. Either it's mainly about the relation between saturation and value, or the curve must not be direct (but rather spiraling or going around edges).
This will likely require further investigation.

Also, this relates to #34, where the image processing functionality should be rendered testable with automatic testing.

[starturtle]

added change (i.e. switch from HSL cone distances to YUV channel distances) to another branch (i.e. feature branch for #34 - first imaging related unit tests)