#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...
Of course, argh.  I am being tremendously boneheaded here.

So the most obvious solution I see is that when we call symmetrizeImage(), we can check if ```image.array.shape``` is odd.  If not, we define ```shape_to_use = image.array.shape + 1``` (instead of just ```=image.array.shape```).  Then we carry out all the steps (```_get_update_rootps_symmetrizing```, ```_generate_noise_from_rootps```) using ```shape_to_use``` instead of ```image.array.shape```.  In the end, if we've input an even-sized image, we'll have a noise field that is larger than the input ```image``` by 1 row and column, so we take a subset of it before adding it to our ```image```.  

This involves drawing the correlation function in a larger image and doing the DFT.  The alternative is drawing the correlation function into an image corresponding to ```image.array.shape```, doing the DFT to get the PS, and then padding the PS image.

Is there some reason to prefer the latter?  (which seems to be what you were suggesting)