Function scheme

docs/app/js/sanddance-app.js:127099–127105

(name, scheme1)

apply(discrete, colors);
apply(continuous, (_)=>interpolateColors(colors(_)));
function scheme(name, scheme1) {
    name = name && name.toLowerCase();
    if (arguments.length > 1) {
        schemes[name] = scheme1;
        return this;
    } else return schemes[name];
}
const SymbolLegend = "symbol";
const DiscreteLegend = "discrete";
const GradientLegend = "gradient";