NA's in future_OM_list object cause obscure errors

[nathanvaughan-NOAA]

Need to confirm that the manual/example documentation makes it clear that all NA's in the future_OM_list object need to be replaced with real values or removed by making them NULL. Lots of NA's are created by the create_future_om_list() function so it is easy for users to leave them in by mistake. It may also be possible to add some code in SSMSE to automatically remove these values, I'm not sure how difficult that would be though.