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)
There's something about the asymmetry of this that troubles me.  The `getSupportedPosAngleConstraints()` says that `UNBOUNDED` is available and yet we need an `allowsUnboundedPositionAngle()` method.  I understand the difference in the two though and I don't know what else to suggest.  I think the need for `allowsUnboundedPositionAngle` will go away anyway when we update the algorithm to take into account vignetting. Maybe we could simply this though.  Perhaps we can just remove the option whenever any instrument is in imaging mode for now?  I think it's worth asking Andy.  If so, we'd need to be able to ask an instrument if it is imaging of course.