STY: Use mbar and Pa for units of pressure

[DWesl]

Description

Fixes https://github.com/NOAA-GFDL/GFDL_atmos_cubed_sphere/issues/350 by using Pa instead of pa and mbar instead of mb as units of pressure. This should make the units of pressure generated by this repository machine-readable. https://github.com/NOAA-EMC/fv3atm/pull/859 makes other units machine-readable

Replaces #355

How Has This Been Tested?

I made similar changes to a version of this repository in global-workflow, which I then compiled and ran. This produced netCDF history files with machine-readable units (specifically the python pint program, though I also checked with UDUnits).

Checklist:

Please check all whether they apply or not

• [X] My code follows the style guidelines of this project
• [X] 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
• [X] My changes generate no new warnings
• [ ] Any dependent changes have been merged and published in downstream modules

[laurenchilutti]

Can you please resolve the conflicts in the file driver/fvGFS/fv_nggps_diag.F90? I think all you will need to do is merge the dev/emc branch into your sty-units-fixes branch and resolve the conflicts from there.