#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...
I found the problem.  FWIW, the issue with odd-sized arrays not working and even ones working had to do with the fact that I was rolling by ```-size/2```, when it should have been ```-(size/2)``` (this has different behavior for odds as for evens: e.g. for size=400, those both give -200, but for size=401, they give -201 and -200, respectively).  So when I was rolling the arrays back to have kx=ky=0 in the corner after finding the symmetrized noise power spectrum, I was actually shifting it too far by one pixel in each direction.

I need to go back over and check that unit tests work after all this refactoring that I've done, and make sure the function names are consistent with our final discussions.  I might change up some of the unit tests as well, to make sure to use both odd and even-sized arrays, and to use order = 4 and higher.  Should be done today, I hope, but will post when done.