Command the Run Engine to pause.

This function is called by ‘pause’ Messages. It can also be called by other threads. It cannot be called on the main thread during a run, but it is called by SIGINT (i.e., Ctrl+C).

If there current run has no checkpoint (via the ‘clear_checkpoint’ message), this will cause the run to abort.

deferbool, optional

If False, pause immediately before processing any new messages. If True, pause at the next checkpoint. False by default.