Implement momentum panning

[zmeeeeeva]

#67

Demo: https://jsfiddle.net/zmeeva/6urw1s9d/

Usage:

let myZoom = zoom()
  .momentum(true)
  .on('zoom', handleZoom)

[curran]

Great concept! I took a quick look through the code and noticed some minor formatting issues.

In your demo, I notice some unexpected behavior:

• If I drag, then stop moving, then release the mouse, the momentum is way higher than I would expect
• If I drag, then stop moving, then release the mouse, I would expect that the momentum should be zero

Perhaps a time component is needed here, as in, when the drag starts, start monitoring the movement with a setInterval or similar, then take the average motion over the last 3 intervals, or something like that.

[curran]

Minor formatting inconsistency

Suggested change

	cancelMomentumTracking();
	cancelMomentumTracking();

[curran]

Suggested change

	var p = select(this).on('dblclick.zoom');
	var p = select(this).on("dblclick.zoom");