saturation: add CIE Yuv and RGB interpolation methods, issue #171
All three methods are linear interpolations towards luminance desaturation of the image, though in different color spaces - yielding slightly different results. The default behavior remains the CIE Lab based interpolation for now. Perhaps in a later release we will also change the default to be the interpolation in linear native space, while adding CMYK support to desaturate that does not lose the separation.
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