Wrong gain selected in get_gain for OSIRIS

[cdfassnacht]

Hi, it looks as though the incorrect gain is being used for the OSIRIS imager. If I look at the get_gain method for the OSIRIS class in instruments.py, it selects the value associated with the DETGAIN header keyword, . For recent files, that has a value of ~5.65. However, in looking at both the header comments and the values given on the OSIRIS instrument page, it appears that the correct gain is 2.15 e-/ADU, which is the value associated with the SYSGAIN keyword. Unless you are using the output of get_gain for something other than converting ADU to electrons, I think that this is a bug that should be fixed.