Resolve "Incorrect SFGs do not raise"
Closes #165 . Modifies 'replace_operation' to handle the case 'sfg.replace_operation(Sink(), "out0")' without raising error. Adds test for "Keep graph_id if possible"
Closes #165 . Modifies 'replace_operation' to handle the case 'sfg.replace_operation(Sink(), "out0")' without raising error. Adds test for "Keep graph_id if possible"