#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 GalSim we often try to give a reference when we introduce some maths.  Could we give a reference for this reciprocity expression, or perhaps derive it based on the assumption of a power law relationship between count rate and flux?

A related comment: I think in the expression used above the alpha is in 'per decade' units. We should possibly be explicit about that.