Update gain_models.py
Created by: GiovanniVolta
Gas xenon gain during cooling phase: 2020-08-26. PTMs at 1.5 kV a part PMT 116 and 195 which were at 1,3 kV. PMTs 354, 362, 386, 427 were off.
Note that during led calibration of August 26th PMTs 264, 272, 285, 300, 308, 311, 313, 316, 325, 339, 356, 359 didn't acquire data. For them I have used the results from 2020-08-23. For PMT 137, which were always off, I have used the result from 2020-07-24.