Deprecation of CopyCorners

[twicki]

Description

After a deep-dive into how our model performs on CPU it became apparent that the CopyCorners codes were underperforming by a large margin. The implementation of those is happening in PyFV3 and therefore the CopyCorerns calls are being deprecated here.

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

[FlorianDeconinck]

We need to mention - if not assert or try/catch in the model - that those are halo=3 tuned. Previous code was as well and that wasn't really said

[romanc]

Looking good. I have two questions:

1. Is stencils/ still the right folder now that corners aren't stencils anymore?
2. Is there any way we can do (simple) tests here in NDSL to ensure corners work correctly?

[FlorianDeconinck]

Let's move this inside pyFV3 and deal with it in NOAA-GFDL/pyFV3#87