Include `value_type` parameter in PropertyMap.copy()

src/graph_tool/__init__.py

@@ -425,9 +425,10 @@ class PropertyMap(object):
         return ("<PropertyMap object with key type '%s' and value type '%s',"
                 + " for %s, at 0x%x>") % (k, self.value_type(), g, id(self))
 
-    def copy(self):
-        """Return a copy of the property map."""
-        return self.get_graph().copy_property(self)
+    def copy(self, value_type=None):
+        """Return a copy of the property map. If ``value_type`` is specified,
+        the value type is converted to the chosen type."""
+        return self.get_graph().copy_property(self, value_type=value_type)
 
     def get_graph(self):
         """Get the graph class to which the map refers."""