#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...
Sigh.  Of course you understand correctly, and what's really sad is that in the little test script I wrote to look at the power spectra while playing around with this whole noise symmetrizing process, I was in fact doing the shifting to get them centered so they would look normal... yet I didn't make the connection that I had to do that when doing the rotation process.  And as you said, tests with n=4 would not show a problem. Since most of my own tests and all the unit tests used n=4, it seemed fine.  I just pushed a fix to use roll2d to get the PS centered properly, and confirmed that higher order symmetry results (a) were screwed up before and (b) are okay now.

However, roll2d only works to center it properly if the NxN image has an even value of N.  If it's odd, then that doesn't work.  I can't just use the InterpolatedImage.shift() method, because that won't do the wrapping around, it'll just pull zeros off the edge.  Is there an obvious solution that I'm missing?