SW-695: P2->P3 Add __lem__ and __iter__ where UserDIct was changed to MutableMapping

CEP/Pipeline/framework/lofarpipe/support/lofaringredient.py

@@ -221,6 +221,13 @@ class LOFARingredient(MutableMapping):
         self._fields = fields
         self._values = {}
 
+    def __len__(self):
+        return len(self._fields)
+
+    def __iter__(self):
+        for field in self._fields:
+            yield field
+
     def __getitem__(self, key):
         # If we don't have the value for this key, but we do have a field with
         # a valid default, return that.