This change enabled CZML and the Entity API to load 3D Tiles tilesets.
Since `Cesium3DTileset` has dozens of options, I only exposed
uri/show/maximumScreenSpaceError to start. We also use the entity
position/orientation (if specified) to compute the tileset.modelMatrix
allowing for moving tilesets that can be tracked with the camera.
Details:
1. New Sandcastle example, `CZML 3D Tiles.html`
2. New class, `Cesium3DTilesetGraphics` for representing a tileset via
the Entity API.
3. New class, `Cesium3DTilesetVisualizer` for creating/managing the
primitives.
4. Add `Entity.tileset` which is an instance of `Cesium3DTilesetGraphics`
5. Specs for everything
Matthew Amato