Adds m5 forecasting kaggle example

[skrawcz]

I pretty much verbatim copied the code from this notebook. It does have a few odd quirks in it -- so if I understood what was going on I removed it, else I left it in. E.g. test_2 is not used...

This is an example that shows:

1. loading data
2. creating a base set.
3. adding more features.
4. creating a training set.
5. fitting model & predicting with it.
6. saving the result.

There are a few other ways one could write some of this code. But as a first pass this seems fine. E.g. we could split up the model training function a bit more. Or we could use the parameterize decorator.

No unit tests have been added, or data quality checks.

Changes

• adds time-series kaggle example

How I tested this

• runs locally

Notes

• brings in warts from notebook that it comes from

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[skrawcz]

add doc string