Correcting array factor away from pointing center

Created by: tikk3r

As in the telecon today, it would be nice to be able to correct for the array factor in a direction away from the delay or reference direction.

I see that #68 (closed) was similar to this, where the direction option was added. Prefactor already corrects for the beam at the pointing center (where array factor = 1 if I'm not mistaken). Attempting to take out just the array factor in another direction (on a phaseshifted dataset) results in

std exception detected: In applying the beam (with invert=true): the metadata of this observation indicate that the beam has already been applied

I split a couple timeslots and channels from a phaseshifted MS that already has the beam taken out at the original phase center by prefactor and uploaded it here: ftp://ftp.strw.leidenuniv.nl/pub/sweijen/test_beam.ms.tar