#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...
Commenting on my own PR because I just had a thought:

Technically, this line that determines how much power to add is the most extreme case.  It follows the pattern from ```_get_update_rootps_whitening```, for which the final PS has to be white and therefore the only option when adding power is to add the same amount to all entries.  However, here, it is *not* the case that the final PS should be white, and therefore what is being done here is conservative (in the sense that it adds too much power in general) but also the simplest possible thing since of course adding a constant trivially preserves the N-fold symmetry.

So then I thought why not figure out for each entry whether ```final_arr``` is above ```ps```, and just add enough to make sure that's the case (or add nothing, if it's already true).  The problem with that, of course, is that since ```final_arr``` was already symmetrized but ```ps``` lacks the symmetry we want, then if we do that, we lose the symmetry of ```final_arr```.  So that is no good!

Then I thought of more complicated things that could maybe do the job of not adding more power than necessary while successfully imposing symmetry (haven't thought them through enough to figure out if they'd really work, but they are definitely make the algorithm quite a bit more complex, which is not necessarily a great idea).  So the question is how much to worry about this? my inclination at this point is to simply note in the comments that this is a conservative choice for the amount of power to add, but leave it as is, and see if anybody who uses it complains.  Any thoughts?  Anybody have an idea for a simpler solution that'll work?