Merge pull request #158 from dolfin-adjoint/dham/_ad_str_fix

Code elsewhere assumes this and it's already true for AdjFloat.

pyadjoint/overloaded_type.py

@@ -308,6 +308,7 @@ def _ad_dim(self):
         """
         raise NotImplementedError
 
+    @property
     def _ad_str(self):
         """Return the string representation of the block variable.