Adding midpoint coloring

[mortonjt]

It would be great if there was support for diverging colormaps (vmin, vcenter and vmax) as done in matplotlib's TwoSlopeNorm. I think this use-case will become increasing common for publishing Empress figures in high impact venues that require fine-tuning of color legends.

To see what I mean, consider the following - I have two different differential abundance profiles that I want to compare, but because of the inability to put them on a common scale, the colors don't align (they are centered at -0.36 and -1.78 respectively when they should both be centered at zero). In my particular case, this is actually a blocking issue.

Also note that this has been raised in emperor before.

biocore/emperor#708

[mortonjt]

zoops, just realized that this is partially implemented here : #484 #521

And has been mentioned in #365 #518 and #529
will follow up with midpoint coloring in a sec