Update z-band zero-points for each amplifier

[djschlegel]

Arjun has measured the following z-band zero-points from photometric good-seeing
frames on the night of 9/10 Feb 2016 as follows:
im4: 26.406
im7: 26.609
im11: 26.556
im16: 26.499
Since it's not measured for the other 12 amps, just use the average of
these four at 26.518 .
These numbers already updated in the IDL mosstat.pro code.

[dstndstn]

Done in 5beea62