#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...
...and this is done.  Good news: it seems to work, based on some checks I did of the intermediate steps of the calculation.  Bad news: the current version of this algorithm to find the minimal PS that has the required symmetry fails if the requested array size is odd (it gives something that can go below the original PS).  What's weird is that it works when we ask for an even-sized array, even though the PS that we actually deal with in that case is also odd-sized.

I need to dig into this peculiar result, but no time for a detailed investigation today, so I will leave this alone for now.  Needless to say there's no point in anyone going through the code at this point because something is wrong.