Color computations

[gwhitney]

I think it might be helpful to add colors as a datatype that can be manipulated in mathjs formulas. There could be functions to convert RGBA values (and/or HSVA values and/or HSLA values) to colors, to take a linear combination of colors, to overlay one color on another, maybe to multiply a color by a scalar (affecting its opacity, I presume; ideally that would mesh with linear combinations). As one simple example, if such formula were allowed in the Grid visualizer, the ten properties that implement "color by last digit" in Grid could be replaced with a single formula along the lines of hsv((a%10)/10, 1, 1). Such formulas might also be usable to streamline ModFill as well.

[gwhitney]

This is being done in #530.