temporary fix

nifty4/operators/linear_operator.py

@@ -271,8 +271,9 @@ class LinearOperator(NiftyMetaBase()):
             raise ValueError("requested operator mode is not supported")
 
     def _check_input(self, x, mode):
-        if not isinstance(x, Field):
-            raise ValueError("supplied object is not a `Field`.")
+        # MR FIXME: temporary fix for working with MultiFields
+        #if not isinstance(x, Field):
+        #    raise ValueError("supplied object is not a `Field`.")
 
         self._check_mode(mode)
         if x.domain != self._dom(mode):