fix OperatorAdapter.__repr__ index out of range

self._trafo \elem {1,2,3}. Therefore, either list needs to have
four items or should be accessed via [self._trafo - 1].
This way seem more readable to me.

Bug can be triggered by creating a FFTOperator `op` and calling
`print(op.inverse.adjoint)`.

nifty5/operators/operator_adapter.py

@@ -69,6 +69,6 @@ class OperatorAdapter(LinearOperator):
 
     def __repr__(self):
         from ..utilities import indent
-        mode = ["adjoint", "inverse", "adjoint inverse"][self._trafo]
+        mode = ["invalid", "adjoint", "inverse", "adjoint inverse"][self._trafo]
         res = "OperatorAdapter: {}\n".format(mode)
         return res + indent(self._op.__repr__())