Add ops for color space transformations

As suggested by @bnorthan in https://github.com/imagej/imagej-ops/issues/355#issuecomment-183882884, I propose to add a `color` namespace with ops such as `rgb2hsb`, `rgb2lab`, `rgb2luv` etc. that have 3-channel, n-dimensional inputs and outputs.

In ImgLib2, we have `net.imglib2.script.color.RGBA` and `net.imglib2.script.color.HSB` so far.

I'm unsure how normalization should be done (e.g. ImageJ 1.x has 8-bit HSB channels, other applications usually use channels normalized to the `[0; 1]` range).

Start with the proposal in issue #520 and the linked discussion in imagej-ops#355, then inspect the mentioned ImgLib2 classes net.imglib2.script.color.RGBA and HSB. The scope still needs decisions about the color namespace, supported transformations, and channel normalization. Done means those choices are defined and the requested 3-channel, n-dimensional operations are implemented.

Written by the indexing model from the issue text.