Finds the LCO instrument gain in electrons per ADU for the current Frame
Description:
This primitive obtains the gain in electrons per ADU for the current frame. It first
attempts to find a value from the ORAC_GAIN header. If this is null or less than 3.0, the primitive
uses a time-dependent default value, and it reports the use of the default.
The gain is returned through an argument.
Notes:
This primitive is suitable for LCO optical instruments.