Utility for transforming `Models` with new auxiliary fields

[glwagner]

The discussion on #3098 had me thinking about how we might allow users to access more model data within forcing or boundary condition functions. The problem we encountered in the past (especially for diffusivity fields) is that dumping everything into the forcing function often caused GPU compilation to fail. That's why forcing functions only have access (for example) to velocities, tracers, and auxiliary fields:

Oceananigans.jl/src/Models/NonhydrostaticModels/nonhydrostatic_tendency_kernel_functions.jl

Line 62 in fad81b0

model_fields = merge(velocities, tracers, auxiliary_fields)

Oceananigans.jl/src/Models/NonhydrostaticModels/nonhydrostatic_tendency_kernel_functions.jl

Line 74 in fad81b0

+ forcings.u(i, j, k, grid, clock, model_fields))

A possible solution to this is a utility with_auxiliary_fields for "rebuilding" a model with new auxiliary fields. I think this would be relatively simple to implement, because the auxiliary_fields don't have any role within a model constructor. Thus we can develop something like

model = with_auxiliary_fields(model, new_auxiliary_fields)

which is quite easy I think, something like...

function with_auxiliary_fields(model::NonhydrostaticModel, new_auxiliary_fields)
    model_properties = OrderedDict{Any, Any}(name => getproperty(model, name) for name in propertynames(model))
    model_properties[:auxiliary_fields] = new_auxiliiary_fields
    return NonhydrostaticModel(model_properties.values...)
end

Then users can run simulations with the "updated" model, putting any fields they like from the "old" model into auxiliary_fields.

[glwagner]

Won't implement anytime soon