Separate gain and exposure time

[ntessore]

When working with maps of effective exposure times (e.g. the EXP image in HLA data), we must still be able to specify an effective gain factor. So I propose to rename the current gain to a new property exp and re-introduce gain as an optional setting with default value 1.

Example use for a SLACS lens with exposure time 2088 and electronic gain 2:

image   =   HST_10886_02_ACS_WFC_F814W_drz.fits[SCI][3565:3764,2169:2368]
exp     =   2088
gain    =   2

[ntessore]

@fabiobg83 Any thoughts on this?

[rbmetcalf]

The meaning of exposure time depends on the telescope.