Use new corners in del2cubed stencil

[romanc]

Description

Follow-up from PR #87, let's use the new corner implementation in del2cubed. The stencil is used in the translate tests and currently emits a deprecation warning.

How Has This Been Tested?

The stencil is called as part of the translate tests. All good as long as they still pass.

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: N/A
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included: N/A
• Targeted model if this changed was triggered by a model need/shortcoming: N/A

[twicki]

Good from my side