ContinuousQuantitativeScale.interpolate() method and interpolator accesor

[anthonime]

Interpolator accessors in Interpolators class (originally d3.interpolate* methods) are hard to implement in Java using generics. They should be wrapped in JSNI implementation of the existing InterpolatorFactory interface but I could not find a way to do it for now. Find a way to implement them, then we can provide the method Scale.interpolate().