fixup! Implement a basic callback system for calling a custom functio…

…n after each iteration.

tardis/simulation/base.py

@@ -295,8 +295,9 @@ def add_callback(self, cb_func, *args):
          The callback ID
         """
         self._callbacks[self._cb_next_id] = (cb_func, args)
+        cb_id = self._cb_next_id
         self._cb_next_id += 1
-        return self._cb_next_id
+        return cb_id
 
     def remove_callback(self, id):
         """