#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.
Yes, Mike, I was doing exactly what you said in the second paragraph - thinking of factoring out a nominal gain conversion that gets applied separately from nonlinearity.  But you're right that it could be combined (again assuming that you don't mind things being out of order - for NIR detectors the order is nonlinearity, then read noise, then gain according to the documents I got from Jason).

Do you agree that the new sentence I suggested in the docstring in https://github.com/GalSim-developers/GalSim/pull/606/files#r19400885 would be a suitable replacement for what's currently there about images having to be in e-, and would properly convey what is intended here about how the function can be used?