Resolve "Move `utils.data_ops` into its own top-level module or subpackage"

This MR moves dantro.utils.data_ops into its own new subpackage, dantro.data_ops. It separates out the individual definitions and brings them into a more maintainable form.

Furthermore, this implements the is_operation decorator.

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
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved

Related issues

Closes #298 (closed), #164 (closed)