#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'm slightly confused as to why the image must be in units of electrons.  The user is free to choose the function they wish, and so could easily adopt one which is able to handle any electron<->adu gain conversion required for whatever specific case they have in mind.