Improve performances of _Fields.__contains__

The default implementation relies on __iter__, which is much
slower and memory consuming than the new implementation which
verifies directly whether a given field exists.

Author: amorison

stagpy/_step.py

@@ -232,6 +232,12 @@ def __iter__(self):
         return (fld for fld in chain(self._vars, self._extra)
                 if fld in self)
 
+    def __contains__(self, item):
+        try:
+            return self[item] is not None
+        except error.StagpyError:
+            return False
+
     def __len__(self):
         return len(iter(self))