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)
Exactly. This is how it is being handled now: if an option is not available at all for the instrument, it does not appear in the combo box. If it is available for the instrument but not for the current configuration, it appears but is greyed out.