add option to list_files to supply path names

[jkingslake]

I could be useful to add an option to list_files so that it can list path names as well as full file names.

It might be just replacing

        return dat_filenames

with

        import os
        return dat_filenames os.path.dirname(dat_filenames)

but this will require changing the usage of list_files elsewhere. Unless, we add an option to determine if pathnames are returned.