Move my_som/domain file specification to usermods_case/frierson/shell_commands

[patcal]

Problem:
Currently, the domain.nc and my_som_file.nc files needed for the FRIERSON compset are hardcoded with fixed system-dependent paths into various XML config files. These should be removed from the config files and the default files should be set in a shell_commands file under the usermods_case directory.

The default should point to files in the users case directory.
Can the default data files be included in the usermods_case directory, which are copied into the case directory at creation?
If not should they be added to the REPO with the other domain/data files?