Detect if preview_namelist needs to be called and avoid calling it.

preview_namelist is called multiple times between "create_newcase" and "case.submit" to make sure user changes make it in to the run, even if users haven't made any changes. Preview_namelists can be expensive. After the first call to preview_namelists, add logic to check if any files have been modified that require preview_namelists to be run again and, if not, don't.