Color domain ordinal #816 fix

[QuinnLee]

This is to fix #816, but not ready to be merged. I want to get feedback first.

d3.scale.ordinal() has a rangePoints while d3.scale.linear() do not. This checks it and sets the color domain.

One problem I realized is how to handle sorting when generating the color domain for ordinal scales. @gordonwoodhull What do you think should happen?
Should the set of _chart.data().map(_chart.colorAccessor());? What about the order? Or should this option not set the domain when scale is ordinal?

Thanks!

[gordonwoodhull]

Hmm, I wonder if we should use the ordering that we use for the x domain:

https://github.com/dc-js/dc.js/blob/a2e765fe72d0cb3101229ac03323c79bfdd6a86f/src/coordinate-grid-mixin.js#L377

I guess that would be consistent, anyway. Maybe in an ideal world there would be an ordering function for each ordinal domain, but right now we order the data, not the keys.

[QuinnLee]

@gordonwoodhull I pushed new changes. I've added

_chart._ordinalColorDomain = function () {
    var groups = _chart._computeOrderedGroups(_chart.data());
    return groups.map(_chart.colorAccessor());
 };