Fix the variance of gain in `segmentation.txt` for  data/comcam https://github.com/lsst-ts/phosim_syseng4/blob/aos/data/comcam/segmentation.txt .  The update of v1.0.6 of phosim_syseng4  included updating the content of data/comcam/segmentation.txt with values  for gain / read noise from obs_lsst LsstComCam instrument. 

However, while for `lsst/segmentation.txt` the previous file  version had  in column 9   no variation for gain (number after 1.16821):

R22_S22_C00 0 2001 0 511 -1 -1 1.16821 0 1000.0 2.0 5.606 0

for `comcam/segmentation.txt` there was a 3% variation (number after 1.4273) 

R22_S22_C00 0 1999 0 508 1 -1 1.4273 3.0 1000.0 2.0 14.3 3.0

which was preserved in the update. This leads to the gain discrepancy at 3% level between phosim-simulated comcam images and  the content of the obs_lsst mapper . 

This ticket is to fix the discrepancy by removing the  variation in gain for `comcam/segmentation.txt` (changing in column 9  "3" to "0", like for `lsst/segmentation.txt` above). 

*Completion criteria*:

Verify that the gain value in phosim-simulated comcam image is identical to the content of obs_lsst LsstComCam , eg. using the attached code. Currently there is a difference in `gain` as a result of the discrepancy as illustrated by the attached output of running python test_gain.py .