Resolve "Extend `count_unique` data operation to operate along dim"

This MR extends the count_unique data operations so that it can be applied along certain dimensions only.

Details

Uses the split-apply-combine approach to apply count_unique along the grouped dimensions.

Anything to double-check?

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • !174 (merged)
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @blsqr

Related issues

Closes #166 (closed)