Automates DIC boundary condition setup for multiple

[jagoosw]

This PR adds a possible way to streamline DIC boundary condition setup when there are multiple realisations of the carbon chemistry, e.g. the example changes:

CO₂_flux1 = 
    CarbonDioxideGasExchangeBoundaryCondition(; 
        wind_speed,
        water_concentration = CarbonDioxideConcentration(; DIC = :DIC1,
                                                           Alk = :Alk1)
    )
CO₂_flux2 = 
    CarbonDioxideGasExchangeBoundaryCondition(; 
        wind_speed,
        water_concentration = CarbonDioxideConcentration(; DIC = :DIC2,
                                                           Alk = :Alk2)
    )

wind = FluxBoundaryCondition(-ρₐ/ρₒ * Cᴰ * wind_speed^2)

boundary_conditions = (; DIC1 = FieldBoundaryConditions(top = CO₂_flux1),
                         DIC2 = FieldBoundaryConditions(top = CO₂_flux2),
                         u    = FieldBoundaryConditions(top = wind))

to

CO₂_fluxes = CarbonDioxideGasExchangeBoundaryConditions(2; wind_speed)

wind = FluxBoundaryCondition(-ρₐ/ρₒ * Cᴰ * wind_speed^2)

boundary_conditions = (; DIC1 = FieldBoundaryConditions(top = CO₂_fluxes.DIC1),
                         DIC2 = FieldBoundaryConditions(top = CO₂_fluxes.DIC2),
                         u    = FieldBoundaryConditions(top = wind))

This is still a bit cumbersome and it would be nice to just do:

CO₂_flux = CarbonDioxideGasExchangeBoundaryCondition(; wind_speed)

wind = FluxBoundaryCondition(-ρₐ/ρₒ * Cᴰ * wind_speed^2)

boundary_conditions = (; DIC1 = FieldBoundaryConditions(top = CO₂_flux),
                         DIC2 = FieldBoundaryConditions(top = CO₂_flux),
                         u    = FieldBoundaryConditions(top = wind))

but currently regularize_boundary_condition(condition...) does not get the field name. I'll keep on thinking of a solution, or if that would be the preferred interface propose the change in Oceananigans.

@johnryantaylor

[jagoosw]

This also retains the current interface CarbonDioxideGasExchangeBoundaryCondition(args...; kwargs...) which falls back to CarbonDioxideGasExchangeBoundaryCondition(1, args...; kwargs...) and uses DIC and Alk