Graceful exit on SIGTERM

[vasdommes]

When SDPB process receives SIGTERM signal, it should exit gracefully.
Possible behaviour:

• Finish current solver iteration
• Write a checkpoint
• Set terminate reason "SIGTERM signal received"
• Exit with SIGTERM exit code (=15)

SIGTERM is used e.g. by SLURM to notify a job of its termintation:

Once a job has been selected for preemption, its end time is set to the current time plus GraceTime. The job is immediately sent SIGCONT and SIGTERM signals in order to provide notification of its imminent termination. This is followed by the SIGCONT, SIGTERM and SIGKILL signal sequence upon reaching its new end time.

https://slurm.schedmd.com/preempt.html