#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.
Well, the formula there assumes that the normalization p'/T' is 1e-/sec, which need not be the case in general. So we could add another parameters that takes in the flux with respect to which it is normalized. I just included that, but the docstring explaining it might need a little more polishing. Also, I have set the default value as 1.0 . Is this something that we shold require the user to specify always?

From the discussion I had with @chazshapiro, the functional form is not given in that reference but is only 'motivated' from the figures in the reference. And there isn't going to be any publication on H4RG detectros any time soon, which will probably justify it more rigorously, but we don't plan to wait that long.

Also, it's the expression for Reciprocity Failure, not for IPC as @rmandelb mentioned earlier.