#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.
Here, you should just be able to do self._a_t_flat_inv = -self._a_t_flat.  

In fact, I would rather just do this in the applyBackward function in the call to `applyCD()`.  Just make the arguments there `-self._a_t_flat`, etc.  I doubt there is a big performance hit for negating the images each time you call that, since I wouldn't think you would be doing that all that often.  And it seems cleaner to have the matrices stored in a single place rather than twice in case we make a function that will modify these matrices at some point.  We don't want to have to remember to also modify the inverses.