#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 were going with Mike's suggestion, which is to not specify the units, just say that it's up to the user to specify the right functional form given whatever units they are working in (i.e., they should choose the NLfunc appropriately for an image in e- or ADU depending on what they choose to pass in).