Change terrainExaggeration on the fly?

1. A concise explanation of the problem you're experiencing.

Is it possible to change the terrainExaggeration without recreating a viewer object?

2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.

3. Context. Why do you need to do this? We might know a better way to accomplish your goal.

4. The Cesium version you're using, your operating system and browser.

Not currently, we have an issue open in GitHub to track that feature: #4342