SourceDB: use projected NCP for gaussian position angle
Created by: tammojan
In the current definition of SourceDB, the position of Gaussians is defined as North over East. Here, North refers to the NCP. However, in both PyBDSF and DPPP, this is effectively interpreted as the 'up' direction in the image, which corresponds to the NCP direction in the phase center. This means that for sources far from the phase center, the position angle is wrong.
This could be fixed by computing, per Gaussian source, a position angle correction (given its position and the phase center of the observation, and the NCP). Projection effects should be taken into account here; perhaps it's necessary to declare in the definition of gaussians that an NCP-projection is used here.
Note that currently, PyBDSF and DPPP are consistent for a given pointing, so when not transferring models from one field to another there is no problem.