#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.
Another point here: `gain_ratio` should not have a default value in C++.  We've mostly instituted the policy that all default arguments are set in python, and everything needs to be explicit in C++.  (There may be exceptions, but not many.)  This helps us catch errors when we change the function signature for something -- things won't compile quietly doing the wrong thing with a default parameter.

But I think I agree with Rachel that the `gain_ratio` feature would be better as a python-layer add on.  Especially since it is just a `im *= gain_ratio` line after the call to `applyCD`.  Simpler to just implement it there.