Update units of SWY precip input rasters

[phargogh]

This PR fixes #1571, which corrects the units for SWY's precip inputs to be mm/month instead of mm/year.

Thanks to @nadinetrahan for pointing this out!

Checklist

• Updated HISTORY.rst and link to any relevant issue (if these changes are user-facing)
• Updated the user's guide (if needed)
• Tested the Workbench UI (if relevant)

[emlys]

Thanks @phargogh!