#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.
No problem.  A good habit to get into is to look through the output of `git diff` before you do a `git commit`.  This will show you any places where you made some temporary edit during development that you don't actually want to commit.  

You can also use `git add -p` to selectively add only a subset of your changes to be committed.  This is useful if you want to leave some edits in your local version but not commit them, because they are just temporary things you did for working on the code (extra print statements, commenting out some tests, etc.) but which you aren't ready to get rid of.