Add way to tell if simulation is running

[garryod]

Code using this library or tests within it may sometimes require knowledge of whether the simulation is during it's setup stage or is actively running (e.g. tickit_task).

Currently there is no way to do this, I propose we set an asyncio.Event after self.setup() in the run_forever method of the MasterScheduler.

[abbiemery]

115 adds ability to shut simulation down. Once this is added we can include tests that spin up e.g. schedulers, assert they have started, send them a shutdown command and assert they exit gracefully.

@abbiemery

[abbiemery]

Also add a way to tell if the simulation is no longer running!