#524 - Implementation of a model for CCD charge deflection in images, following Antilogus et al (2014)

This branch is ready for review by a wider group I think, although merging the branch is not highly urgent.

On #524 , @danielgruen and I have added a module (`galsim/cdmodel.py`) to describe charge deflection in CCD pixels, also known as the "brighter-fatter" effect, following the model of Antilogus et al (2014), see http://arxiv.org/abs/1402.0725.

The effect is applied to images at the C++ layer via a new `BaseImage` member function, `applyCD()`.  We chose the constness etc. settings in the way we thought made sense, but comments welcomed.  The Python module, and the classes which store the different charge deflection models, then use this interface to apply the effect.

@danielgruen has put some unit tests in place, and is already using the code to perform tests of the effect's impact on weak lensing measurements.  However, as ever, suggestions for improved docstrings and unit tests would be welcomed on this PR.
I don't like this function being a member of the BaseImage class.  It's not something that needs access to the internal details of the BaseImage class.  In fact, the implementation of the function only accesses thing through public methods.  So that's a pretty good indication that this function ought to be an external function, not a method.

IMO, this belongs in a separate module similar to the hsm module.  This is how you treated this in the python layer, requiring an `import cdmodel` to access the functionality.  In C++, this should also be in a cdmodel subdirectory as we did for hsm.