- add fitted_ attribute to MultiNormalizer when fit is called

[chefPony]

Description

This PR solves issue #680

Checklist

• Linked issues (if existing) MultiNormalizer always raise NotFittedError, even when it was actually fitted #680
• Amended changelog for large changes (and added myself there as contributor)
• Added/modified tests
• 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

[jdb78]

Good catch. Thank you!