Define the all_instances function in the future as well

wrappers/modelverse.py

@@ -1003,3 +1003,7 @@ def connections_between(model_name, source_element, target_element):
 def define_attribute(model_name, node, attr_name, attr_type):
     """Create a new attribute, which can be instantiated one meta-level below."""
     raise NotImplementedError()
+
+def all_instances(model_name, type_name):
+    """Returns a list of all elements of a specific type."""
+    raise NotImplementedError()