Cleanup: Rename `DefaultGrain` to `DefaultGranularity` where missed

[courtneyholcomb]

Description

Missed the classes & file names when renaming this field. Not very important since users won't see this, but thought it would be helpful to have consistency for development.

Checklist

• [x] I have read the contributing guide and understand what's expected of me
• [x] I have signed the CLA
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [ ] I have run changie new to create a changelog entry