Add versions argument to Simulation

[nikhilwoodruff]

Fixes #147

[nikhilwoodruff]

New changes:

1. Drop support for Hugging Face in policyengine.py as discussed.
2. Download the actual dataset version (after data package changes add this) using a new format:

data="gcs://bucket/[email protected]"

3. When no version is specified, log the version of the dataset we used and warn the user that we're using this version.
4. Added pip install prompt when model versions don't match.

[nikhilwoodruff]

@anth-volk added the changes from our in-person review just now.

[nikhilwoodruff]

@anth-volk I also changed the version syntax because I think it's cleaner to have a standardised model_version and data_version property pair.

Old (this PR): data=gcs://bucket/filename.h5@version + model_version="1.0.0"
New (this PR): data=gcs://bucket/filename.h5 + data_version=version + model_version="1.0.0"

[anth-volk]

Loving the work on this PR @nikhilwoodruff. Had a few thoughts and a couple questions in the comments. Additionally, wanted to ask if you could add relevant tests for new methods and one test to ensure that the function that adds the most recent version when its version arg is None does so properly.

[nikhilwoodruff]

@anth-volk addressed your comments, and added tests for the new functionality.

[anth-volk]

One minor blocking issue: Optional-typed fields still need a default value to avoid validation errors when the field isn't passed. Otherwise, looking forward to getting this over the line today!

[anth-volk]

Very excited to get this code in @nikhilwoodruff! It's been a pleasure reviewing.