#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 see what you mean, Mike.
I was thinking in terms of people wanting to test effects of nonlinearity / reciprocity failure on weak lensing, where they will want an image without and with the effect, but that's certainly contrary to our typical approach of having the image methods just modify the image.  SO I guess we should be consistent with our norm, and people who are using this for testing will have to explicitly copy the image before modifying it if they want to keep it for the sake of comparison.