Use a local copy of Rgb2lab to plot colormap guide [backport to 1.4.x]

[jenshnielsen]

Closes #3442 and #3685

This uses a local rgb2lab converted to avoid depending on scikit image when building the docs as suggested by @mdboom . Rather than implementing our own I have copied the one from scikit-image with a few modifications.

[tacaswell]

Is all of that code strictly needed? It looks like we really only need rgb2lab and I would lean towards pulling the minimal amount of code needed to make this work.

[jenshnielsen]

I guess that there are two different options. Pull the minimal amount of code or pull the code with the minimal number of changes. I went for the later solution to make resynchronisation with the original as easy as possible, but I am happy to strip out the sections that we don't need if you think that is better.

[tacaswell]

That is a very good point that I had not considered.

I don't have a good sense the long-term maintenance burden trade off here.

[jenshnielsen]

I will write up another PR with the minimum and then we can see how that looks

[jenshnielsen]

@tacaswell @mdboom any thoughts on which of this and #3737 is the most clean approach. I would like to get rid of this misplaced branch

[jenshnielsen]

Closing replaced by #3737