lab method
Creates chalk with foreground color defined from lab color space parameters. https://en.wikipedia.org/wiki/CIELAB_color_space#CIELAB
Implementation
Chalk lab(num l, num a, num b) {
var xyz = ColorUtils.lab2xyz(l, a, b);
var rgb = ColorUtils.xyz2rgb(xyz[0], xyz[1], xyz[2]);
return makeRGBChalk(rgb[0], rgb[1], rgb[2]);
}