REL-2090

Added code to make migration from old way of representing parallactic angle mode to new way to preserve in update. Now if the old data model had the PositionAngleMode set to MEAN_PARALLACTIC_ANGLE (the way that the parallactic angle mode was previously represented), we make sure that the position constraint is also set to PARALLACTIC_ANGLE.

It is worthwhile to see if we can get rid of PositionAngleMode, which may no longer be needed. I will look into this.
Not a big deal but a minor tweak that does the same thing w/o the explicit `null` check: `"MEAN_PARALLACTIC_ANGLE".equals(pam)"`