Add an "overlay" for variables specific to NorESM and CESM

[mvertens]

Summary: Add an "overlay" for variable defaults so each model keeps only its own differences

Previously, NorESM's aerosol/chemistry plotting defaults were written directly into the shared lib/adf_variable_defaults.yaml. That made the shared file drift from upstream and hard to maintain. This PR adds a small feature so a run can lay a second, short defaults file on top of the shared one — overriding only the variables that differ, and leaving everything else untouched.
In particular, the NorESM overlay file - adf_variable_defaults_noresm - will contain NorESM aerosol and surface emission variables whereas the CESM overlay file adf_variable_defaults_cesm will contain CESM aerosol variables.

IMPORTANT:
The NorESM/CESM aerosol variable defaults were moved out of the shared adf_variable_defaults.yaml into per-model overlay files. As a result, defaults_overlay_file is required for any run whose variable list includes those aerosol variables:

• NorESM runs must set defaults_overlay_file: adf_variable_defaults_noresm.yaml
• CESM runs must set defaults_overlay_file: adf_variable_defaults_cesm.yaml
  Without it, the derived aerosols won't be generated (their derivable_from/derivation_formula and aerosol_zonal_list now live in the overlay), and the remaining aerosol vars lose their plotting defaults.

Changes:

• lib/adf_obs.py — after loading the shared defaults, optionally read a defaults_overlay_file and apply it. Each variable in the overlay replaces that variable's entry in the shared file; variables not listed keep their shared settings. A plain filename is looked for in lib/; a full path is used as-is.
• lib/adf_variable_defaults.yaml — removed the NorESM-specific entries (~705 lines) so the shared file holds only common defaults again.
• lib/adf_variable_defaults_noresm.yaml (new) — NorESM's overlay (its aerosol/chemistry differences).
• lib/adf_variable_defaults_cesm.yaml (new) — CESM's overlay, for the same mechanism.
• config_noresm_template.yaml — turns on the NorESM overlay via defaults_overlay_file.
• CONFIG_REFERENCE.md — documents the new defaults_overlay_file setting and how it differs from defaults_file.

Testing:
A full NorESM ne16 run loads the shared file + NorESM overlay, with the aerosol variables getting their overlay settings and producing tables/plots as before.

[mvertens]

@adagj - this PR should address the issue you raised regarding how to keep CESM and NorESM aerosol and surface emissions separate and support both. I'm happy to try to merge in your changes if you want as a possible next step - if you point me to your branch.