Add gain information

[mpaillassa]

Here are information about the gain for each survey:

• CFHT: 1.67 e-/ADU reference
• DES: 4 e-/ADU reference
• Euclid_VIS: 3.1 e-/ADU reference
• HSC: 3e-/ADU reference
• HST: 1e-/ADU reference
• Rubin: I did not find a value. From here it seems to be ~0.7-1.0 e-/ADU.

[mpaillassa]

A small precision for Rubin. As we discussed on 10/28, we can take 1e-/ADU as gain value as it is the one used in the imSim simulations.

[aboucaud]

Hi @mpaillassa, now that the gain is a supported value, you should open a specific PR to add the gain to every survey (YAML file) and link it to this issue.