Docs require skimage

[mdboom]

Three examples now require it, for the rgb2lab function:

users/plotting/colormaps/lightness.py
5:from skimage import color

users/plotting/colormaps/Lfunction.py
9:from skimage import color

users/plotting/colormaps/grayscale.py
6:from skimage import color

We should probably just subsume rgb2lab -- skimage is a fairly heavy dependency just for this.

[jenshnielsen]

Yes, I was trying to fix all the Sphinx warnings on Travis but never got round to this one. It is even more problematic since this part of skimage depends on scipy too.

I was planing to just deploy the dependencies with miniconda, but skimage also depends on matplotlib so we could easily create circular dependencies.

[mdboom]

Yeah -- but for just converting RGB to LAB, we shouldn't need all that, see, for example:

https://stackoverflow.com/questions/13405956/convert-an-image-rgb-lab-with-python

[jenshnielsen]

Cool that is probably a better solution given the issues above.

[jenshnielsen]

fixed by #3737