#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 thought of that, but then I thought otherwise because

(1) this is completely new functionality and I thought that (as for other stuff we've put into GalSim) we might find we want to change around the user interface once people have used it a bit.  So I thought maybe it wasn't worth adding quite so many bells and whistles yet.

(2) for a lot of simple sims people assume gain=1 so none of this ADU vs. e- stuff matters.

Do you disagree on (1)?