Convert H2OLNZ units to ppm by volume

[chengzhuzhang]

Description

Resolve comments #868 (comment)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have added tests that prove my fix is effective or that my feature works
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[chengzhuzhang]

@tangq this PR converts units from g/kg to ppm by volume for both Q and H2OLNZ as requested. Note that H2OLNZ had already brought to g/kg before the units change happening in zonal_mean_2d_driver.py. The results can be view here. Could you please review the code change and the results? Thanks!

[tangq]

The changes Look good to me. Thanks, @chengzhuzhang

[chengzhuzhang]

Thank you @tangq