Allow users to specify absolute position angle

Still to do: documentation (don't know where this should go yet).

For users: specify OrientationIsAbsolute=True in skymodel (will propagate to sourcedb and sourcedb blob) to specify that the Gaussian orientation (a.k.a. position angle) is oriented with respect to the North Celestial Pole. Specifying OrientationIsAbsolute=False (the default, and the behavior until now) means the orientation angle is with respect to the North at the phase center. For sources that are far from the phase center (e.g. in calibration) or if the NCP is in the field of view this can make a large difference.

I have a separate python script for computing the absolute orientation given the relative one (and a phase center), that should to to LSMTool...