Solves bug when calling ConjugateGradientPoissonSolver without specifying preconditioner

[tomchor]

See #3831 (comment)

[glwagner]

Hmm, I think ImmersedBoundaryGrid is defined by ImmersedBoundaries, which isn't imported until after Solvers:

Oceananigans.jl/src/Oceananigans.jl

Lines 210 to 227 in 99d3c1f

	include("Solvers/Solvers.jl")
	include("OutputReaders/OutputReaders.jl")
	include("DistributedComputations/DistributedComputations.jl")
	# TODO: move here
	#include("ImmersedBoundaries/ImmersedBoundaries.jl")
	#include("MultiRegion/MultiRegion.jl")
	# Physics, time-stepping, and models
	include("Coriolis/Coriolis.jl")
	include("BuoyancyModels/BuoyancyModels.jl")
	include("StokesDrifts.jl")
	include("TurbulenceClosures/TurbulenceClosures.jl")
	include("Forcings/Forcings.jl")
	include("Biogeochemistry.jl")
	# TODO: move above
	include("ImmersedBoundaries/ImmersedBoundaries.jl")

A bit of refactoring is needed to import ImmersedBoundaries before Solvers. So I think if we want to resolve this without all that refactoring we should define the constructor that depends on ImmersedBoundaryGrid separately (either in ImmersedBoundaries or Model.NonhydrostaticModels)

[tomchor]

yeah I think you're right!

[glwagner]

I can fix this if you want

[tomchor]

I can fix this if you want

Thanks, yeah feel free to jump in!

[ali-ramadhan]

This might be a duplicate of PR #3830 but looks like both will be superseded by PR #3847 which is doing the refactoring necessary for this fix.