Add support for lazy and eager broadcast expressions

A common pattern that is seen in the ClimaAtmos diagnostics, and a desirable capability for unit tests and performance, is something this:
```julia
if isnothing(out)
return TD.dry_pottemp.(thermo_params, cache.precomputed.ᶜts)
else
out .= TD.dry_pottemp.(thermo_params, cache.precomputed.ᶜts)
end
```
What we are trying to do is this: when `out` is `nothing`, we want to allocate a new `Field` and return it. When `out` is a preallocated `Field`, we just want to use it. It would be really nice for ClimaCore to support this in a single `out .= something` expression.

Start with the ClimaAtmos diagnostic pattern shown in the issue, especially the `out .= TD.dry_pottemp.(...)` expression and its `out === nothing` alternative. Determine how ClimaCore's broadcast expressions currently handle allocation and preallocated `Field` outputs. Done means one `out .= something` form supports both lazy allocation and eager reuse, with behavior suitable for unit tests and performance.

Written by the indexing model from the issue text.