Add Support for Multiple Pressure Level Requests in 'create_filename'

[MclTTI]

Summary of Changes:

1. Function Update: The create_filename function has been updated to support multiple pressure level requests. It now handles cases where levelout is a list of pressure levels, formatting them properly and appending the unit of measure accordingly.
2. New Validation Function: A new function validate_levelout_get_unit has been added to the config.py module. This function checks whether the levelout specified in the configuration file is valid and returns the appropriate unit of measure.
3. Integration with create_filename: The output of the new validation function is used in create_filename to manage cases where the user requests multiple pressure levels, provided as a list.
4. Updated User Config Template: The config.tmpl file has been updated to explicitly indicate that users can specify multiple pressure levels as a list.

Docstrings have been added where necessary.