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.