wip: implicit vertical smagorinsky diffusion

[haakon-e]

Purpose

To-do

Content

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• I have read and checked the items on the review checklist.

[haakon-e]

This change is part of the following stack:

• rft: eddy diffusivity docstrings #4107
  • wip: implicit vertical smagorinsky diffusion #4173 ◀

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