#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 not quite sure if the image can be in units of ADU. The unit conversions can be taken care of while specifying NLfunc but since the functional forms are fixed by us for Reciprocity Failure, the gain will have to become one of the parameters or the exp_time must include the gain factor.