Cesium3DTileSet should expose overall bounding volume

[mramato]

I wanted to fly the camera to a 3DTileSet, but there doesn't seem a way to get it's bounding volume. I ended up with the below code (not sure if it's correct). A properly like tileset.boundingVolume would probably be ideal.

var bb = tileset._root._tileBoundingBox;
var bs = BoundingSphere.fromCornerPoints(bb.northeastCornerCartesian, bb.southwestCornerCartesian);
return camera.flyToBoundingSphere(bs);

[lilleyse]

@JoshuaStorm Would you like to take a look at this?