#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.
Jason's document has this expression and it is not referenced. I was looking at the relevant papers for the detectors (HAWAII H2RG) and came across these two:
1. http://www.jstor.org/stable/pdfplus/10.1086/658282.pdf,
2. http://www.jstor.org/stable/pdfplus/10.1086/661793.pdf.

But these still quite don't have the expression we use.