#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.
A thought related to the one voiced about ADU images above.  Rather than force users to pre-multiply an image by the gain before using these routines, we could include a gain keyword argument that includes this in whatever operation is being performed.  This would be slightly more efficient than doing the two operations separately.