some tweaks to phydms_testdivpressure

[jbloom]

1. In the script itself, don't use file as a variable name. Replace with fname or something that isn't a Python reserved keyword.
2. Right now the names in the final output file are funny with the directory name is something like phydms_results/ as the files get split at this underscore. Also wasn't clear to me why both file and name were needed, but maybe they are.
3. Update the documentation to show a model comparison file with the actual output format.