Anisotropic biharmonic diffusivity closure

[glwagner]

This PR implements an anisotropic biharmonic diffusivity closure.

Typically this would be used with zero vertical viscosity / diffusivity, and paired with a constant Laplacian vertical diffusivity. For these, we can use tuples introduced by #517.

I hope to implement an example that uses this feature in a PR coming fairly soon.

Still needs:

• docs

[codecov]

Codecov Report

Merging #518 into master will decrease coverage by 1.55%.
The diff coverage is 27.86%.

@@            Coverage Diff             @@
##           master     #518      +/-   ##
==========================================
- Coverage   66.17%   64.61%   -1.56%     
==========================================
  Files          67       68       +1     
  Lines        1898     1950      +52     
==========================================
+ Hits         1256     1260       +4     
- Misses        642      690      +48

Impacted Files	Coverage Δ
src/Oceananigans.jl	75% <ø> (ø)	⬆️
..._implementations/constant_isotropic_diffusivity.jl	100% <ø> (ø)	⬆️
...lementations/anisotropic_biharmonic_diffusivity.jl	0% <0%> (ø)
src/TurbulenceClosures/closure_tuples.jl	50% <100%> (-50%)	⬇️
...mplementations/constant_anisotropic_diffusivity.jl	100% <100%> (ø)	⬆️
src/TurbulenceClosures/TurbulenceClosures.jl	33.33% <26.31%> (-66.67%)	⬇️
src/TurbulenceClosures/closure_operators.jl	66.66% <33.33%> (-13.59%)	⬇️

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