`BackgroundField` is a confusing name

[glwagner]

It's not a Field. The docstring for the struct admits as much:

Oceananigans.jl/src/Fields/background_fields.jl

Lines 33 to 41 in 7a4b3f0

	"""
	BackgroundField{F, P}
	Temporary container for storing information about `BackgroundFields`.
	"""
	struct BackgroundField{F, P}
	func:: F
	parameters :: P
	end

and this temporary struct is always thrown away to build a "real" field:

Oceananigans.jl/src/Fields/background_fields.jl

Lines 63 to 64 in 7a4b3f0

	regularize_background_field(LX, LY, LZ, f::BackgroundField{<:Function}, grid, clock) =
	FunctionField{LX, LY, LZ}(f.func, grid; clock=clock, parameters=f.parameters)

The problem is that the name leads to confusing patterns like

Oceananigans.jl/examples/tilted_bottom_boundary_layer.jl

Line 98 in 7a4b3f0

B_field = BackgroundField(constant_stratification, parameters=(; ĝ, N² = 1e-5))

where B_field is not a field. Indeed, later on when we need a "real" field we have to write

Oceananigans.jl/examples/tilted_bottom_boundary_layer.jl

Line 178 in 7a4b3f0

B∞ = model.background_fields.tracers.b

Can we think of a better name? For example BackgroundFieldInfo is one. That's sorta clumsy though. Maybe we can come up with something more concise and elegant.

cc @tomchor @hdrake @liuchihl because you are using this feature

[hdrake]

Maybe just BackgroundFieldDefinition?

[glwagner]

Not bad...