colors: provides a color gradient in (hue, chroma, luma) (HCY') space.
colors: provides the original ColorGradient as RGBColorGradient as well.
colors: added a global toggle, namely oUF.useHCYColorGradient, to select which algorithm oUF.ColorGradient uses.
Merge pull request #200 from Adirelle/color_gradient

Provides a color gradient based on hue/chroma/luma (HCY').