Add warnings for MultiNormalizer

[jdb78]

Checklist

• [x] Linked issues (if existing)
• [ ] Amended changelog for large changes (and added myself there as contributor)
• [ ] Added/modified tests
• [x] Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install. To run hooks independent of commit, execute pre-commit run --all-files

Make sure to have fun coding!

[jdb78]

Relates to #858