Some cluster tuning.  First go at an opponent surround.

Random wiring model, hacked up for special case where we only use one cone of each class and assumption of uniform field.

Probably need to add some noise after the opponency in the general case.  Not sure I've done the conceptually correct opponency for the one cone case.

Signed-off-by: David Brainard <brainard@psych.upenn.edu>