external file path formatting tied to OS

[kallejahn]

When fileio.setup_external_filepaths() saves the external file paths, it is formatting based on the OS. I create models on Windows, but then often run them on Linux, so that is creating backslash issues.

I've added .replace('\\','/') on lines 509 and 513 in fileio.py as a fix, but not sure if that's what you'd want long term?

    if not relative_external_paths:
        external_files = [os.path.normpath(os.path.join(model.model_ws,
                                       model.external_path,
                                       filename_format.format(i))).replace('\\','/') for i in file_numbers]
    else:
        external_files = [os.path.join(model.model_ws,
                                       model.external_path,
                                       filename_format.format(i)).replace('\\','/') for i in file_numbers]