fig config diagnostic merging; print only on root

[juliasloan25]

Purpose

This PR does 2 distinct things:

1. Fix merging of diagnostics specified in the coupler and atmosphere. Previously, atmos_config_dict["diagnostics"] was overwritten by the coupler config_dict["extra_atmos_diags"]. Now these additional diagnostics are appended, so we output all diagnostics.
2. Initialize the ClimaComms communication context immediately after creating the config_dict, so all printing happens on root in the MPI case

[akshaysridhar]

Thanks @juliasloan25 . Can confirm following this change that diagnostics other than the default set are parsed correctly.

[akshaysridhar]

@juliasloan25 Would splitting up the 2 commits help with this ? It would be helpful to have at least the diagnostic fix as part of the main branch. (I've manually included the diagnostic update commit for now)

[juliasloan25]

@juliasloan25 Would splitting up the 2 commits help with this ? It would be helpful to have at least the diagnostic fix as part of the main branch. (I've manually included the diagnostic update commit for now)

Sorry for the delay - there was a bug with the diagnostic fix that only appeared with some configs. It is fixed now in this PR