photometric zero-point from AG

The obsproc working group would like to suggest that we measure the photometric zero-point in each AG exposure.  This will allow us to measure the median zero-point and rms during an PFS exposure, which will be very useful for us to do an initial quality assessment at the summit in real-time.  We consider this functionality crucial for the future PFS science operation.  The color-term to translate the Gaia photometry into the AG system needs to be estimated.