Only store the relevant data in the stix pickle file

[samaloney]

Currently have to update if the vis object is changed - if we create vis object from data can always use the same file.

         Ok I gess stix_vis.pkl has been updated? Let use > this for the moment. I think what we should do is only > store the data in the file and create the visibility here that > way it won't change if we change the vis object.

Originally posted by @samaloney in #67 (comment)