Add signal handler for ALPINE

When a SLURM job reaches its time limit, the job controller sends it a signal (SIGTERM) to indicate that time is up. Depending on the configuration, the program might then be allowed a short time extension during which it can terminate normally. If the program does not terminate on its own before this extension expires, the controller kills the process (SIGKILL).

ALPINE runtime consists primarily of a timestepping loop. Even if it does not finish all the time steps requested by the user, there is still useful information to be obtained from the timer data. This MR adds a signal handler to ALPINE for both SIGTERM (for SLURM) and SIGINT (to allow the user to terminate with ^C) such that the mini-apps can terminate gracefully and leave useful output behind even if there isn't enough time to complete all the requested time steps.