#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 have made changes so as to input the arguments both as a list or a tuple. I still have the args!=None ( actually it's equivalent one now) to allow for the possibility that the function has no parameters to be specified. 

There are 3 cases: NLfunc takes no parameters, 1 parameter whose types could be int, float or even a tuple and multiple parameters as a list or a tuple. Calling examples for each of the three cases are included in docstring.