operator.py: Raise not-implemented in matmul call

Contrary to other overloaded operators, the __matmul__ call apparently needs to raise an exception instead of returning NotImplemented. This behaviour can be seen for example in:

psp = ift.RGSpace(512)
m = ift.MaskOperator(ift.Field.from_raw(psp, np.random.randint(0, 2, 512)))
m(ift.from_random("normal", m.domain).val) is NotImplemented

which should be invalid and raise an exception but previously returned True instead.

nifty6/operators/linear_operator.py

@@ -175,7 +175,7 @@ class LinearOperator(Operator):
         from ..linearization import Linearization
         if isinstance(x, Linearization):
             return x.new(self(x._val), self(x._jac))
-        return self.__matmul__(x)
+        return self@x
 
     def times(self, x):
         """Applies the Operator to a given Field.