Exporting only long-form boundary condition constructors?

[ali-ramadhan]

Currently the Oceananigans module exports

Flux, Value, Gradient, NormalFlow,
FluxBoundaryCondition, ValueBoundaryCondition, GradientBoundaryCondition,

so there's some redundancy (perhaps we should export one set to reduce namespace pollution) but a minor practical issue is that the Flux type conflicts with the popular Flux.jl package (I guess I'm the only one using them together right now but there may be more in the future?). There's also some inconsistency in exporting NormalFlow but not NormalFlowBoundaryCondition.

In deciding on what to export for the user interface (see #1132) I'm wondering what do people think about only exporting the long-name version, e.g. FluxBoundaryCondition instead of Flux.

I think this will have a few benefits:

1. Lower probability for conflicts. Flux is one example, but Value and Gradient are pretty generic terms so I wouldn't be surprised if they conflict with exports from other packages future users may want to work with.
2. Scripts might read more intuitively, e.g. because you say "a flux boundary condition" and not "a boundary condition of type flux".
3. If we decide to export the complete set of boundary conditions we could do it by exporting PeriodicBoundaryCondition without having to worry about conflicting with the Periodic topology.

X-Ref: #1132

[glwagner]

I'm fine with this. Also helps with #1119 .