Support "flipping" continuous color maps

[fedarko]

So that e.g. viridis could go from yellow to purple rather than purple to yellow. This would add some more variety to the color options present in Empress.

The changes here would mostly involve the Colorer.js module within Empress, as well as some additional changes to the HTML to involve adding a UI element (checkbox?) that allows users to toggle whether or not the color map is reversed. I think this might be as simple as changing [min, max] to [max, min] within this block of code:

empress/empress/support_files/js/colorer.js

Lines 200 to 206 in a76ebf2

	var nums = _.map(split.numeric, parseFloat);
	var min = _.min(nums);
	var max = _.max(nums);
	var interpolator = chroma.scale(this.color).domain([min, max]);
	_.each(split.numeric, function (n) {
	scope.__valueToColor[n] = interpolator(parseFloat(n));
	});

This seems to be how the Chroma.js docs recommend doing this, at least -- see the section around here, right around where it says "To reverse the colors ..."

For reference: brought up by @kwcantrell this morning. We talked about reversing color maps in the context of Qurro a while back at biocore/qurro#277, but I haven't got around to doing that yet :(

[gwarmstrong]

Can I grab this one?

[kwcantrell]

thanks @gwarmstrong! This will a nice feature to have.

[fedarko]

(Closed by #423.)