Callback fixes

[charleskawczynski]

I think the ClimaAtmos branch is getting close to covering all of the edge cases, so I think it's a good time to start incorporating the post-callback fixes into ClimaTimeSteppers. The edge cases that we missed included:

• Accounting for when !isnothing(T_imp!) && !iszero(a_imp[i, i]) is false
• Krylov evaluations of f! (T_imp!) with something other than U/x-- which requires an additional post_implicit! call
• Calling post_explicit!(U, p, t_imp) instead of post_explicit!(U, p, t_exp), and doing this after temp is computed (to preserve the existing behavior).
• Finally, computing T_imp using the U from before the post_implicit! call, which requires injecting this into the callback (we add two closures to avoid unnecessary additional assignments).

Latest build, with (I think) all of these fixes is here:

https://buildkite.com/clima/climaatmos-ci/builds/13544