Create a helper function and Sandcastle example for CWB

[ggetz]

Cesium World Bathymetry(CWB) has just been released.

Just like createWorldTerrainAsync for Cesium World Terrain and other common datasets, we'll want to include a helper function for it.

We'll also want to include a new Sandcastle to demo the dataset. Here's an existing one I've put together which can be adapted to use the new helper function.