Precip units fix

[nicolejkeeney]

Description of PR

Fixes bug in monthly precip unit conversions. I had to change some logic in core/data_load.py... I am hoping I didn't introduce any new bugs, and tested this myself, but could use some careful review on the units here.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

n/a

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [n/a] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules

[nicolejkeeney]

Fixed the units stuff, evaluating unrelated LOCA2 issues in another PR