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)
The note below about "MUST have an entry in the PosAngleConstraintRegistrar" should be deleted?  It looks like that is going away in this PR.