Allow for DateTime clocks

[ali-ramadhan]

Should be useful for LESbrary simulations whose states might actually have an associated date and time from the real world.

But I'm not sure if we should merge this PR as DateTime from Base.Dates only has millisecond precision so you could get small errors in actual model.clock.time after many iterations (LESbrary simulations will probably run for millions of iterations).

Two small packages that might help us here
https://github.com/JeffreySarnoff/TimesDates.jl
https://github.com/FugroRoames/Chrono.jl

[glwagner]

I agree that something like this would be pretty convenient.

[codecov]

Codecov Report

Merging #632 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #632      +/-   ##
==========================================
+ Coverage   78.03%   78.16%   +0.12%     
==========================================
  Files         118      118              
  Lines        2363     2395      +32     
==========================================
+ Hits         1844     1872      +28     
- Misses        519      523       +4     

Impacted Files	Coverage Δ
src/Simulations.jl	93.10% <0.00%> (-2.73%)	⬇️
src/OutputWriters/checkpointer.jl	92.00% <0.00%> (+0.33%)	⬆️

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[ali-ramadhan]

So I modified this PR to support more general AbstractTime clocks. So now clocks work if you pass in a DateTime from Base.Dates (with millisecond precision) but also if you pass in a TimeDate from TimesDates.jl (with nanosecond precision) as they're both subtypes of Dates.AbstractTime.

Oceananigans does not need to depend on TimesDates.jl but I added it as a test dependency so we can make sure TimeDate always works as I think it will be useful for LESbrary simulations.

[glwagner]

could also call this “tick!” But looks good

[ali-ramadhan]

Love it!