Anderson Acceleration script doesn't work with rpointer updates

### Brief summary of bug

After rpointer files were updated to include year-month-date in their names the logic applied in Anderson Acceleration doesn't work. This is ultimately a change that needs to be done in the Anderson acceleration code, but I'm creating a CLM side issue because it influences MIMICS usability in more recent tags and is a blocker for using MIMICS after ctsm5.2.027. I'd like to resolve this before the CLM6 release.