Allow `None` value for --[in|out]put-port

This is probably a band-aid covering up for earlier poor design choices, but --input-port=None and --output-port=None are now allowed in calls to the get_*_factory() functions in factory.py. If None, a default value is used in subsequent calls to initialize the respective factories.

This fixes broken geomag-filter processing that arose when a None value was used to initialize factories after the --input-port and --output-port options were added in the previous merge request.