Add warning if GPU model grid Nx, Ny are not multiple of 16

[ali-ramadhan]

Add a warning if trying to create a GPU model with a grid where Nx or Ny are not a multiple of 16.

Some GPU kernels still use the hard-coded Tx = Ty = 16 for calculating thread-block layouts. See PR #308 for why.

Even if we were to get rid of the hard-coded Tx, Ty there will always be weird grid sizes like 17x33x47 that you can run no problem on a CPU, but might not be able to create a perfect thread-block layout for without having extra threads that do nothing, but extra threads can do bad stuff like apply a boundary condition twice if we're not careful.

Apologies to @beta-effect who was burned by this in the past.

[glwagner]

<3 for using grid.Nx, etc.

[glwagner]

Hmm, actually do you want to move our hard-coded values of Tx, Ty to models.jl, and then reference them in the model constructor? Might be clearer.

[codecov]

Codecov Report

Merging #398 into master will increase coverage by 1.36%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #398      +/-   ##
==========================================
+ Coverage   64.62%   65.99%   +1.36%     
==========================================
  Files          23       23              
  Lines        1405     1532     +127     
==========================================
+ Hits          908     1011     +103     
- Misses        497      521      +24

Impacted Files	Coverage Δ
src/models.jl	92.53% <100%> (-0.32%)	⬇️
src/time_steppers.jl	72.41% <0%> (-0.92%)	⬇️
src/boundary_conditions.jl	84.25% <0%> (+2.9%)	⬆️

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[codecov]

Codecov Report

Merging #398 into master will increase coverage by 5.82%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #398      +/-   ##
==========================================
+ Coverage   64.62%   70.45%   +5.82%     
==========================================
  Files          23       23              
  Lines        1405     1682     +277     
==========================================
+ Hits          908     1185     +277     
  Misses        497      497

Impacted Files	Coverage Δ
src/models.jl	91.3% <75%> (-1.56%)	⬇️
src/output_writers.jl	77.71% <0%> (+0.54%)	⬆️
src/time_steppers.jl	73.89% <0%> (+0.55%)	⬆️
src/turbulence_closures/TurbulenceClosures.jl	57.69% <0%> (+1.44%)	⬆️
src/boundary_conditions.jl	86.11% <0%> (+4.75%)	⬆️
src/Oceananigans.jl	90.9% <0%> (+7.57%)	⬆️
src/fields.jl	54.86% <0%> (+8.03%)	⬆️
src/utils.jl	64.44% <0%> (+14.44%)	⬆️
src/poisson_solvers.jl	76.41% <0%> (+36.41%)	⬆️

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