better behaviour for factor=0.

nifty/operators/scaling_operator.py

@@ -69,7 +69,10 @@ class ScalingOperator(EndomorphicOperator):
 
     @property
     def inverse(self):
-        return ScalingOperator(1./self._factor, self._domain)
+        if self._factor!= 0.:
+            return ScalingOperator(1./self._factor, self._domain)
+        from .inverse_operator import InverseOperator
+        return InverseOperator(self)
 
     @property
     def adjoint(self):
@@ -81,5 +84,7 @@ class ScalingOperator(EndomorphicOperator):
 
     @property
     def capability(self):
+        if self._factor==0.:
+            return self.TIMES | self.ADJOINT_TIMES
         return (self.TIMES | self.ADJOINT_TIMES |
                 self.INVERSE_TIMES | self.ADJOINT_INVERSE_TIMES)