Fixed numpy error when using w.past in w_ipa: optional argument seg_ids would either be a None type or a numpy array, but checking if it was None failed when it was specified.
Fixed numpy error when using w.past in w_ipa: optional argument seg_ids would either be a None type or a numpy array, but checking if it was None failed when it was specified.


Former-commit-id: eb667a81865d455bdf1858909c88b570675df42c
westpa/westpa