#529 - imposing isotropy or at least N-fold symmetry on correlated noise

This pull request is for some small additional functionality in the handling of correlated noise, which is parallel to the whitening functionality that was already in place but involves imposing N-fold symmetry on the noise (without making it white).  Here N is some even integer >=4, with the default choice being 4, likely the lowest order of symmetry necessary to avoid weird shear systematics due to anisotropic correlated noise.

In general, the amount of noise that has to be added is substantially less than what has to be added to make correlated noise in COSMOS images fully white.  So that would be the main motivation for doing this instead of whitening.

Based on some discussion on #529, the ```applyWhiteningTo()``` method has been changed to ```whitenImage()```, parallel to the new ```symmetrizeImage()``` (though ```applyWhiteningTo()``` still works for now).   Mike put the new symmetrizing option into config, and moved the existing whitening option in config from being an attribute of the real galaxies to being specified at the same time as the noise in the final image.  (Mike, should we update the config wiki once this is merged?  Or did we decide the wiki was going to correspond to the latest tagged version?)

@emhuff , this should be helpful for you...
Okay, I implemented this.  However, to address the end of your last comment, once I've done this there's no need to do this whole ```delta_pow``` thing at all.  I am nearly positive that this procedure gives the minimal final PS that (a) has the desired symmetry and (b) is >= the input power.  So I got rid of that code chunk.

As an aside, here's an image of the noise PS when imposing 20x symmetry on the COSMOS noise:
![20xsymm](https://cloud.githubusercontent.com/assets/1454906/3516759/e3f8ee0e-06e4-11e4-9a23-8f5bd17b2f5e.jpg)
Very pretty!

Now when I do this, I find that the original COSMOS noise had variance of 1e-5, the 4x symmetrized one has variance of 1.05e-5, and the 20x symmetrized one has 1.1e-5.  (The whitened one has 6.4e-5.)  So this is successful in minimizing the amount of added noise.  Obviously, the COSMOS noise was pretty isotropic to begin with - if I shear it a lot then the numbers aren't quite as nice, but they still aren't too bad.