getTileDataAvailable() handling for terrain providers

This is a tile loading optimisation for cameras that start near the globe based on advice from Kevin.  It is currently only implemented for the Cesium terrain provider which knows in advance which tiles are present.
I suspect this is to get the total seconds as part of the network profile?

I'd still rather not have these here; it is yet another place we need to remember to update.  Perhaps just separate Sandcastle examples would be OK - long-term, we may have a label for examples not included in the final release but used for testing and profiling.  Or perhaps there is a way another way to get a time interval from the network profile?  Or just not submit anything for now.