Fix cell_diffusion_timescale with zero tracers

[ali-ramadhan]

No description provided.

[glwagner]

Might be better to use dispatch here (on the case where the kappa-tuple has length zero) for performance, clarity, and julianism!

[glwagner]

Could define a function cell_viscous_time_scale that returns this

[ali-ramadhan]

I thought of defining cell_viscous_timescale and cell_tracer_diffusion_timescale so that cell_diffusion_timescale just returns

min(cell_viscous_timescale, cell_tracer_diffusion_timescale)

but then cell_tracer_diffusion_timescale would have to return Inf instead of nothing and it seemed a bit messy with lots of one-liner functions and duplicated dispatch signatures so I just refactored cell_diffusion_timescale to just dispatch on whether closure.κ is an empty named tuple or not.

I added tests to make sure cell_diffusion_timescale will work for all closures when buoyancy=nothing, tracers=nothing.

[codecov]

Codecov Report

Merging #691 into master will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #691      +/-   ##
==========================================
+ Coverage   78.05%   78.22%   +0.16%     
==========================================
  Files         120      120              
  Lines        2447     2466      +19     
==========================================
+ Hits         1910     1929      +19     
  Misses        537      537              

Impacted Files	Coverage Δ
...rbulenceClosures/turbulence_closure_diagnostics.jl	100.00% <100.00%> (ø)

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