Upgrade lineChart.interpolate -> lineChart.curve in a backward-compatible way

[gordonwoodhull]

Since d3 has replaced d3.svg.line.interpolate with d3.line.curve, we should too. But we have the opportunity to do it in a mostly backward-compatible way.

First, we can create the new lineChart.curve() which takes a curve object, but leave lineChart.interpolate(), still taking a string but instantiating the right curve object based on the old names of interpolations. We should deprecate interpolate using dc.logger.deprecate, and remove it in 3.1 or later.

@kum-deepak, I think we can leave tension the way you have implemented it in _interpolateWithTension, but instead of silently dropping tension when it's not applicable, we should warn the user in the browser console, "Tension specified for a curve that doesn't support it, ignored"

[kum-deepak]

This is a good idea. It has some additional benefit. See below.

I also wanted to change the behavior of tension a little bit. In the previous version the default value was 0.7 while this made the output look different, after changing to 0 it looks similar. So, in all likelihood interpretation of tension has changed within D3.

I was thinking to set value of default tension to null instead of 0. That way if someone did not set it we do not try to apply it.

This also has one benefit that one can apply the tension to the d3 curve function before passing it to chart.curve (like chart.curve(d3.curveCardinal.tension(0.5))). As tension will not be reapplied, it will work as expected.

[gordonwoodhull]

That's great. Maybe we should deprecate tension too, since that looks like a more expressive, idiomatic way to customize the curve.

[kum-deepak]

Great, will make these changes and raise a PR.

[gordonwoodhull]

fixed by #1381