#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 think this paragraph needs some caveating.  Like "If you are using this to implement nonlinearities in the readout electronics, then...."  Or I would even be happy with removing this paragraph entirely, since the function is really much more general than this paragraph implies.  Users may have a variety of reasons for using this function that are not just the physical reason you have in mind here.