Ensure run is closed only after sending final metrics & events

[alahiff]

Currently metrics & events can be sent after the run is closed, which ideally shouldn't happen.

[kzscisoft]

@james-panayis and I implemented this as part of the move to Dispatcher by default when the termination trigger is called the user is no longer allowed to add additional items to the metrics and events queues. The dispatcher will then continue until the queues are empty. I assume as the Run object has to wait for the dispatcher (via join) that the run would indeed be kept open as well.

https://github.com/simvue-io/client/blob/1c68897d5639c37f4a7a3939857db97f2c9d44a3/simvue/dispatch.py#L78

https://github.com/simvue-io/client/blob/1c68897d5639c37f4a7a3939857db97f2c9d44a3/simvue/dispatch.py#L124

[kzscisoft]

Tested by tests @james-panayis has added in #292