Graceful shutdown

[Modularius]

Summary of changes

Detects a SIGINT signal from the OS and performs a graceful shutdown.

trace-to-events and digitiser-aggregator

For trace-to-events and digitiser-aggregator, both the producer thread, and the main thread listens for SIGINT.

• When the producer thread hears it, it closes the MPSC channel and flushes all messages (eventlists and frames respectively) before terminating the thread.
• When the main-thread hears it, it waits for the producer thread to end, then terminates.

nexus writer

When the signal is received, the main (and only) thread blocks, moves any nxs files in the move_cache queue, then terminates.

Instruction for review/testing

General code review.

Tested on simulated data.

Closes #270.