#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 wouldn't get so close to zero.  Anything under `sys.float_info.min` will have inaccuracies when dividing, so probably better to have this be the guard.  Also, I'd just make this a warning rather than an error.  The user may be ok with some nan's in the resulting image where division by zero occurred.