In the last 2D DRP technical telecon on 2021-01-04, we agreed to use the position angle, POSANG, rather than the rotation angle, ROTANG.

Propose to add the following as the definition for POSANG:

PosAng Intended Position Angle of the PFI (degrees).
This corresponds to the angle measured clockwise
looking along the positive PFI_Z axis (ie the
telescope pointing direction),
from the North Celestial Pole to the PFI_Y axis.

Martin Reinecke in particular: is this acceptable?

I'm not (yet) 100% sure about the conversions necessary to go from this value to the one we need in shuffle and the fiber assigner, but the definition is clear and unambiguous, so we should be able to manage.

Because this modifies the datamodel of PfsConfig files, merging it will break all existing data repos. We need a transition plan and/or backward compatibility.