Determine when to end stream

[HagaiHargil]

Currently rPySight doesn't know when to end reading from the data stream. During online acquisition it's less of a problem, but during replay the rendering window simply hangs instead of gracefully exiting.

[HagaiHargil]

I need to do some Future shenanigans here - the thread that spawns the call_timetagger.py task needs to be polled during the looping and checked whether the TT is already done.

[HagaiHargil]

I managed to fix the replay portion of this issue by checking whether the Arrow stream is closed. The question still remains on how to signal rPySight that a live acquisition is over. When the GUI will function again we might want to add a button there.