#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...
>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 think it's the other way around - the roll2d works straightforwardly to centre the PS when it is an odd dimensioned array, since then it makes sense that the central point of the array (the centre of the central pixel) is kx=ky=0.

It is when the array is even that you have more trouble.  Then you need to account for the fact that the array is not symmetric around the kx=ky=0 point.  The solution, I think, is to make the array PS array odd dimensioned by copying the array values at the lower left 2 edges [that is, once the array has been circularly rolled by (N/2,N/2)] to corresponding diametrically opposed locations on the upper right 2 edges.  Rather like in the `__init__` method of the `CorrelatedNoise` class.