Add .chunk operation for xarray

This MR adds the xr.Dataset.chunk/xr.DataArray.chunk method as default operation.

Can this MR be accepted?

Seems like default/"piped" operations are not explicitly tested, so I did not add a test. I can do so, if required.

• Implementation ready
• Tests added or adjusted (not needed)
• Documentation extended or updated (automatically documented)

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

• Ready for merging
  • Pipeline passes without warnings
  • squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @

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