Make WorkElement attributes explicitly native str type.

Make WorkElement.namespace and WorkElement.operation constant member
properties that are initialized as native str whether the client
initializes through the constructor or the class deserialize().

Refs #228

src/gmx/workflow.py

@@ -397,10 +397,10 @@ def __repr__(self):
 class WorkElement(object):
     """Encapsulate an element of a work specification."""
     def __init__(self, namespace="gmxapi", operation=None, params=None, depends=()):
-        self.namespace = to_string(namespace)
+        self._namespace = str(to_string(namespace))
         # We can add an operations submodule to validate these. E.g. self.operation = gmx.workflow.operations.normalize(operation)
         if operation is not None:
-            self.operation = to_string(operation)
+            self._operation = str(to_string(operation))
         else:
             raise exceptions.UsageError("Invalid argument type for operation.")
 
@@ -423,6 +423,16 @@ def __init__(self, namespace="gmxapi", operation=None, params=None, depends=()):
         self._name = None
         self._workspec = None
 
+    @property
+    def namespace(self):
+        assert isinstance(self._namespace, str)
+        return self._namespace
+
+    @property
+    def operation(self):
+        assert isinstance(self._operation, str)
+        return self._operation
+
     @property
     def name(self):
         assert isinstance(self._name, (str, type(None)))