#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.
All the tests seem to use `base_flux = 1`.  You should have something that checks other values of base_flux.  Also, the tests with exp_time = 200 don't really test that it is accurate.  The tests that the math is right all use exp_time = 1.  This should also be tested with another value.