Correct prox for LInfty functional (#1615)

I believe the radius was wrong in the prox of the LInfty functional. I have checked that with pen and paper. Is there a way to verify this otherwise? Perhaps another human could have a look?

odl/solvers/nonsmooth/proximal_operators.py

@@ -1494,7 +1494,7 @@ def __init__(self, sigma):
         def _call(self, x, out):
             """Return ``self(x)``."""
 
-            radius = 1
+            radius = self.sigma
 
             if x is out:
                 x = x.copy()