Feature/ocsadv 23

These changes address the GUI issues brought up in OCSADV-23, disabling the unbounded position angle option for MOS imaging and custom mask FPU observations (and FPU = None).

Also, this fixes a bug with respect to the parallactic angle controls appearing when they shouldn't.
(See REL-2089: http://jira.gemini.edu:8080/browse/REL-2089)
Indeed. I have no idea, either. I thought that people might be concerned if they didn't see the options listed at all, as they might figure that those options were not available for the instrument, which is why I thought that the disabling was a good idea.