Add regression test notebook/script

[rameshvs]

Common tasks:

• Train a simple sklearn model and save results to MLFlow
• Generate visualizations with matplotlib and plotly
• Simple dask data processing

[gg314]

Perhaps this fits best inside orbyter-demo as a Python notebook.

I could imagine including a test script in orbyter-cookiecutter but it may not apply to / work with all images. And embedding it via orbyter-docker doesn't seem like a good idea.