Keep in mind that the [Packable](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Core/Packable.js) and [PackableForInterpolation](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Core/PackableForInterpolation.js) interfaces are implemented by many different Core types.  I'm fine with tweaking names, but we need to make sure things stay consistent for all types and that the names in the `Packable` interface make sense in relation to each other.  I personally would just leave the Packable stuff as is and if we want to add a `fromArray` that just calls `unpack` I'm fine with that too.  `writeElements` can just go away.