[REF!] type hint the codebase

[baggiponte]

Issue tracker for "100%"-ish annotations in code.

This can be quite time consuming (some modules, such as preprocessing, are 2k+ LoC) but is a good exercise to familiarise with the code base.

• [ ] Train-test split
• [ ] Window splitters
• [ ] evaluation.py
• [ ] feature_extractors

And more to come...