initial commit for creating a demo

[wir963]

@TracyYXChen this PR is to create your requested demo. Please play around with the code and see what you think and provide any suggestions.

[TracyYXChen]

@wir963 Thanks! It works perfectly. I have successfully applied it to my simulation dataset.
Yet there is one big issue and one minor issue:

• The estimation of exposures on the held-out dataset, i.e. could you please include stm-heldout*.R in the demo, too?
• Please add package r-argparse to the environment file

[wir963]

@TracyYXChen could you take a look at my changes and see if they make sense to you. If they do, please approve and I'll merge it in.