Resolve "Add DAG-based generic plot function using xarray FaceGrids"

This MR implements the generic facet_grid function for the external plot creator. It wraps xarray.plot functionality creating xarray.FacetGrid objects.

Can this MR be accepted?

• Implementation ready
  • Implement the facet_grid function
• Tests added or adjusted
  • Test facet_grid basic functionality: Are plots generated without errors?
• 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 by @blsqr

Related issues

Closes #139 (closed)

#140 (closed), utopia!85 (merged), utopia#106 (closed)