Use lazy broadcasted expressions

[Sbozzolo]

Right now, users have to write code like:

        if isnothing(out)
            return copy(state.c.ρ)
        else
            out .= state.c.ρ
        end

This is very ugly. This pattern also is not amenable to fusing kernels.

[charleskawczynski]

Changing the interface here would be ideal: compute!(out, args...) would become compute(args...), and users would no longer have access to ClimaDiagnostics' backed storage (which they currently do).

If users were required to use LazyBroadcast, then this problem goes away.