SeawaterBuoyancy with constant temperature or constant salinity

[glwagner]

Resolves #581

[codecov]

Codecov Report

Merging #595 into master will decrease coverage by 2.56%.
The diff coverage is 66.15%.

@@            Coverage Diff             @@
##           master     #595      +/-   ##
==========================================
- Coverage   77.03%   74.46%   -2.57%     
==========================================
  Files         125      121       -4     
  Lines        2155     2181      +26     
==========================================
- Hits         1660     1624      -36     
- Misses        495      557      +62

Impacted Files	Coverage Δ
...lence_closure_implementations/smagorinsky_lilly.jl	92.85% <100%> (ø)	⬆️
...nce_closure_implementations/blasius_smagorinsky.jl	50% <100%> (ø)	⬆️
src/Buoyancy/linear_equation_of_state.jl	29.41% <33.33%> (-39.82%)	⬇️
src/Buoyancy/Buoyancy.jl	63.15% <61.11%> (-36.85%)	⬇️
src/Buoyancy/nonlinear_equation_of_state.jl	70.83% <70.83%> (-29.17%)	⬇️
src/Buoyancy/seawater_buoyancy.jl	70.58% <73.33%> (-12.75%)	⬇️
...ure_implementations/leith_enstrophy_diffusivity.jl	32.25% <0%> (-66.13%)	⬇️
src/Operators/interpolation_operators.jl	59.52% <0%> (-4.77%)	⬇️

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