#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...
If I understand correctly, the power spectrum (PS) in the array `ps` has its (central) ux=uy=0 value in the lower left pixel of the array, due to the usual way that DFT outputs are stored and as followed by the `numpy.fft` routines used here.

Is this also the point around which the PS is being rotated, or is it the centre of the array?  I don't see any centroid shifting, and maybe this needs to be accounted for.  (By chance, for the fourfold rotation I think you end up with either exactly or almost exactly the same thing because of the symmetry of the way the PS is stored...!)