Description
This is the follow-up PR of NOAA-GFDL/NDSL#240. In there, we deprecated the old corner-code in NDSL. Here , we enable a better-perfoming verison of CopyCorners written in plain python.

This also streamlines the copy_corners_[X/Y]_nord implementation to not re-implement the full logic but re-use the logic implemened.

📣 This changes the previously errenous implementation of the nord check and now - as was in fortran - implements the comparison to 0 instead of the current_nord. This oversight was never caught because the loop iteration and the value were always far enough away from each other, but this is true to what the original model-code did.

How has this been tested?

Ran orchestrated & non-orchestrated regression tests (CopyCorners / FvTp2d and Del6VtFlux)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• Targeted model if this changed was triggered by a model need/shortcoming