Creating transition on Selection from other Transition

[tomwanzek]

The type constraint on the interface augmentation Selection.transition(...) for generating a transition from an existing transition should probably be relaxed.

I.e. more like:

export interface Selection<GElement extends BaseType, Datum, PElement extends BaseType, PDatum> {
   ...
        transition(transition: Transition<BaseType, any, any, any>): Transition<GElement, Datum, PElement, PDatum>;
    }

Note that, the returned Transition is still a transition on the elements of the Selection from which it was created.

As an example, where this strategy is in use, see Brush & Zoom II