CLAHE filtering of RGB images

[aoles]

Add option to filter color images in the luminance domain.

For this we need an efficient C implementation of RGB <-> Lab converter functions, for details see here.

[dschneiderch]

have you thought about using Rvision or ropencv as a dependency since these interface with opencv? Rvision has access to the color space conversion functions in opencv. currently only support
'BGR', 'BGRA', 'GRAY', 'BayerBG2BGR', 'BayerGB2BGR', 'BayerRG2BGR', 'BayerGR2BGR' but this could be relatively easily extended to rgb->lab, rgb->hsv, et al. swarm-lab/Rvision#19
Looks like you could also access the CLAHE algorithm of opencv if you know how to expose c++ functions to Rcpp.

I spent an afternoon (trying to) replicate some of my python based plantcv workflows and I found the lack of color space conversions the most limiting.