_Fields.__contains__ fails on invalid field

This method now only masks `MissingDataError` instead of any
`StagpyError`.  In particular, checking whether an invalid field name is
present results in a `UnkownFieldVarError`.

Author: amorison

stagpy/_step.py

@@ -280,7 +280,7 @@ def __iter__(self):
     def __contains__(self, item):
         try:
             return self[item] is not None
-        except error.StagpyError:
+        except error.MissingDataError:
             return False
 
     def __len__(self):

tests/test_field.py

@@ -4,10 +4,10 @@
 import stagpy.phyvars
 
 
-def test_field_unknow(step):
+def test_field_unknown(step):
     with pytest.raises(stagpy.error.UnknownFieldVarError):
         step.fields['InvalidField']
-    assert 'InvalidField' not in step.fields
+        assert 'InvalidField' in step.fields
 
 
 def test_field_missing(step):