Clamp Path to Ground

This was [requested on the forum](https://groups.google.com/d/msg/cesium-dev/8R-SkHsbP78/eTVwYj1vBAAJ), to be able to clamp a `Path` to the ground, so clamping the path here for example:

https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/?src=CZML%20Path.html&label=CZML

Initially I thought this might be trivial, as you'd only need to pass the "clampToGround: true" since the `PathGraphics` just uses a polyline anyway, but digging a bit into it, it seems like it might not be so straightforward, since a Polyline and a clamped Polyline are two different primitive types?

Start with the CZML Path Sandcastle example and trace how PathGraphics uses a polyline. Compare that path with the clamped Polyline primitive mentioned in the issue, then determine the changes needed for Path to support ground clamping. Done means the example path can be clamped to the ground without breaking the existing path behavior.

Written by the indexing model from the issue text.