#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.
In the context of using it for the so called 'classical' voltage-linearity, it is only natural that the image is in the units of electrons. The user would need to know for what value of the charge stored in the pixel was the nominal gain calculated. However, if the user is using this for someother purposes, then it is upto the user to decide the units. The new docstring is revised to make this clear.