AnimationCollection cleanup

`AnimationCollection` refactor (now `Tween` and `TweenCollection`) for #1188.  Tweens are used throughout Cesium for camera flights, camera bounce in 2D, scene morph, and the selector widget.

As some of us discussed, I suggest we mark this `@private` since it has a ways to go before it is the "CSS Animations for Cesium properties" it was originally envisioned to be.  Some ideas are in the new roadmap (#1830).  I encourage someone to take this over post 1.0 so we have really nice mouse over effects and so on.

Please start looking at this.  There are just a few small things left:

* [ ] Mark `@private` (and update CHANGES.md)
* [ ] Remove Sandcastle examples
* [ ] Add tests
This is "private" because it is used by `TweenCollection` below.  It is more like C# `internal` or a C++ `friend`.  We do this elsewhere, I think it is fine.