#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 thought we decided not to say the image had be in electrons, but rather point out that the user should be careful about making sure the non-linear function provided is appropriate for whether the image is in e- or ADU.