Reduce unnecessary specializations

[charleskawczynski]

This PR removes some unnecessary specializations.

One thing I realized is that we were compiling quite a few copyto! methods because of all the slab / column calls, which forwarded to additional copyto! methods that then needed to be compiled. I don't think that this actually buys us anything.

I used https://timholy.github.io/SnoopCompile.jl/dev/tutorials/pgdsgui/ to apply some de-specializations. Here's one result:

copyto!(dest::ClimaCore.DataLayouts.AbstractData, bc::Union{Base.Broadcast.Broadcasted, ClimaCore.DataLayouts.AbstractData}) @ ClimaCore.DataLayouts ~/dev/CliMA/ClimaCore.jl/src/DataLayouts/copyto.jl:5
(397 specializations) # before

copyto!(dest::ClimaCore.DataLayouts.AbstractData, bc::Union{Base.Broadcast.Broadcasted, ClimaCore.DataLayouts.AbstractData}) @ ClimaCore.DataLayouts ~/dev/CliMA/ClimaCore.jl/src/DataLayouts/copyto.jl:5
(... specializations) # now (pending...)

I also applied strip_space to the broadcasted objects in the stencil kernels, since this could reduce specializations on getindex of the broadcasted object.

I also added a singleton type into the datalayouts, which we can use to keep our generic code while avoid overspecializing on specific datalayouts.

[charleskawczynski]

Superseded by #2041, #2042 and #2043