Add implementation

[charleskawczynski]

The idea here is that, when we start using LazyBroadcast in ClimaAtmos, we'll use the Absent type, so that broadcasting will entirely skip arithmetic operations against them. In this implementation, I've also defined a helper that we should be able to use, skip_materialize, that we can use to even completely elide operations in cases where we have expressions like @. x += Absent()-- we can make materialize!(dest, ::Base.Broadcasted{<:AbsentStyle}) = nothing here.

For example, in CliMA/ClimaAtmos.jl#3540, we'll be able to use:

viscous_sponge_tendency_ρe_tot(ᶜρ, ᶜh_tot, ::Nothing) = Absent()

There are a few advantages of doing this:

• We can optimize the @. x += Absent() case (which will happen often early on before we begin fusing tendencies)
• We'll hopefully avoid more issues with inference failure when broadcasting over multiple structs and a Field ClimaCore.jl#2065
• Dropping many Absent objects during broadcasted construction could also help with compile times

I added LazyBroadcast to the test dependencies here for convenience, but we could remove it.

Credit to @dennisYatunin, for most of the implementation in CliMA/ClimaTimeSteppers.jl#264

[charleskawczynski]

By the way, everything that is not in this PR was automatically generated using https://github.com/CliMA/ClimaPackageTemplates.jl (I've made some updates):

using Revise;
import ClimaPackageTemplates as CPT
CPT.make_package(;
    pkg_dir=pwd(),
    pkgname="AbsentTypes.jl"
)

(after cloning an empty repo from github)

[charleskawczynski]

This is working, so I'm going to merge and change the name to NullBroadcasted, it's technically a broadcasted object, so I think that it's more descriptive, and I think null is a more common name / concept than absent.