Operator: cleanUp(): Set parent=None after detatching from graph.
Slot: Added an assertion to enforce relationship between connected operators: Must be siblings or direct parent/child
Cleanup: Remove graph arg from calls to Operator.__init__
Operator: Improved an assertion message
OperatorWrapper: Fixed a problem with slot removal logic.
When an output slot is about to be removed, ensure that backpropagate_values doesn't interfere with proper disconnection during removal.