F extend yaml model

Summary

See #68 (closed). Introduce a new keyword for yaml-models that allows insertion of Records and File objects.

In the course, setting (default) values of properties has been implemented and, more importantly, setting values of properties of records directly.

Note that this breaks one of the existing tests: values of properties can be different from dicts now.

Maybe related: #35

Focus

I added annotations in the commits.

Test Environment

There are new unit tests showing how to test the new keywords.

Check List for the Author

• All automated tests pass
• Reference related Issues
• Up-to-date CHANGELOG.md
• 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
• 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 spezifications? Are they satisfied?

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