Handle naming convention change for RRFS files

[timdonohue-aerology]

Note: while modest in its scope, it's my first public contribution so may want to lend extra scrutiny (and apologies for any bad etiquette).

Around 4/19/24, RRFS URL naming convention changed on AWS (directory structure doesn't appear to have changed, just the file names):

• The CONUS domain changed from conus_3km to conus
• For ensemble members, m0x was added between date and product

Changes are confined to rrfs.py, where URLs are now generated for all combinations of domain and ensemble member naming conventions. The SOURCES dictionary should remain compatible with find_grib from core.py, though includes up to 4 aws_i keys.

[blaylockbk]

Looks great! Thanks for the update :)