[WIP] Additional pkz+rdg_var mods

[jjuyeonkim]

Description

• A few more rdg_var and pkz modifications.
• Because reg_rdgas_div_gravity is useful in both fv_dynamics and dyn_core, I moved the neg_rdgas_div_gravity into its own file to be imported (rdgas_adjust.py).
• Added some changes to init_utils and initialize_baroclinic related to rdg_var and grav_var.

How Has This Been Tested?
Basic unit tests pass, but more testing is forthcoming.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• Targeted model if this changed was triggered by a model need/shortcoming

[fmalatino]

Ignore last comments, thought about it more.