Should we give the option to add background fields to model fields?

[tomchor]

We already do that in some diagnostics by default like

Oceanostics.jl/src/FlowDiagnostics.jl

Lines 95 to 111 in 87b2973

	if model isa NonhydrostaticModel
	if ~(model.background_fields.velocities.u isa Oceananigans.Fields.ZeroField)
	u += model.background_fields.velocities.u
	end
	if ~(model.background_fields.velocities.v isa Oceananigans.Fields.ZeroField)
	v += model.background_fields.velocities.v
	end
	if ~(model.background_fields.velocities.w isa Oceananigans.Fields.ZeroField)
	w += model.background_fields.velocities.w
	end
	if ~(model.background_fields.tracers.b isa Oceananigans.Fields.ZeroField)
	b += model.background_fields.tracers.b
	end
	end

I think it's a useful feature, but maybe it shouldn't be applied to all cases. For example in an Eady-like set-up, where the domain is horizontally periodic and the velocity has a background gradient this will create discontinuity in x and/or y if the velocity gradient is implemented as a background velocity. So maybe this should be an opt-in feature in the diagnostics.