#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.
This last sentence seems wrong.  The function is expected to be able to operate on a numpy array.  Not an Image instance.  I think the right meaning is "The functional form of `NLfunc` must be such that it correctly handles whatever units (ADU or e-) the original Image is specified in."  

Also in the previous sentence, I think image -> array would be clearer and in line with the use of "array" earlier in the paragraph.