More well-defined export statement / user API

[glwagner]

This PR rewrites the top-level export statement for Oceananigans.jl. Mainly it omits many names from the export statement that we do not intend to be part of the user API.

Resolves #364.
Resolves #313.

[codecov]

Codecov Report

Merging #399 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #399     +/-   ##
=========================================
+ Coverage   71.69%   71.79%   +0.1%     
=========================================
  Files          23       23             
  Lines        1413     1411      -2     
=========================================
  Hits         1013     1013             
+ Misses        400      398      -2

Impacted Files	Coverage Δ
...closures/rozema_anisotropic_minimum_dissipation.jl	23.95% <ø> (ø)	⬆️
src/Oceananigans.jl	100% <ø> (ø)	⬆️
src/turbulence_closures/TurbulenceClosures.jl	56.25% <ø> (ø)	⬆️
...ures/verstappen_anisotropic_minimum_dissipation.jl	52.54% <ø> (ø)	⬆️
src/output_writers.jl	77.71% <ø> (ø)	⬆️
src/poisson_solvers.jl	98.4% <ø> (+0.78%)	⬆️
src/fields.jl	62.82% <100%> (+0.79%)	⬆️
src/turbulence_closures/smagorinsky.jl	67.79% <100%> (ø)	⬆️
src/equation_of_state.jl	100% <100%> (ø)	⬆️

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[glwagner]

@ali-ramadhan merge if you approve.