#552 Detector non-linearity and reciprocity failure

A basic method to apply any desired non-linearity on an Image has been included to begin with. The non-linearity can be specified by a LookupTable or as a function, possibly with parameters, that can act on NumPy arrays. In addition, it also includes a feature to correct for reciprocity failure. A basic unit test to test these functionalities have also been built.
> I feel like the places we use a gain are a bit fast and loose compared to the steps that Jason laid out for doing 
> everything correctly. 

Ditto.

>  I kind of think if you are doing all these very careful tracking of detector effects, you should just have the e- -> 
> ADU step be explicit and never let GalSim handle the gain for you along the way.

Yes, in light of your previous point I am inclined to agree with this.