Add type hints to models.py

Summary

Introduce type hints in models.py. #144 (closed)

Focus

This MR adds many type hints to models.py, but is not complete. Also, probaby not all type hints are fully correct and will have to be checked, in particular whether really all Optional arguments are actually optional, i.e. are allowed to be None.

Test Environment

Regular test environment.

Check List for the Author

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• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

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