Run example notebooks with nbval

[rolyp]

Use nbval to verify that our example notebooks/use cases execute properly. Might want to switch to pre-built Stan model as to run in JupyterLab, some nest_asyncio.apply() incantation is required which seems to make the notebook hang when it runs with nbval.

• Git hook that clears notebook outputs before comparing? May not be required with nbval – see ptype