Add support for real-time events and custom signal handlers

[gvoskuilen]

Today, the only real (wall) time feature SST supports is the --exit-after command line option. We would like to add support for heartbeats and checkpoints on a real time interval rather than simulated time. In addition, we want to make it possible to register both user-supplied signal handlers and custom real-time events. We envision this being useful for debug, among other use cases.

Our solution is as follows:

1. Add a real-time management class (RealTimeManager) to unify signal handling and real-time event handling. The class would manage setting SIGALRM for real-time events and would invoke handlers in response to signals (current list: SIGUSR1, SIGUSR2, SIGINT, SIGTERM, SIGALRM). The default handlers would be the same as today.
2. Add a base class and default handlers for RealTimeActions to trigger on signals.

• SIGUSR1, SIGUSR2 - default same as today (status output) but could be changed by user to a custom handler
• SIGINT, SIGTERM - terminate after generating a checkpoint. Shutdown today is immediate and causes MPI error messages - would delay to sync point for graceful shutdown.
• SIGALRM - would be passed to real-time actions which could be a mix of default (e.g., checkpoint, heartbeat, exit) and user-supplied

3. Add ELI types for the new RealTimeAction to allow user libraries to register signal handlers and recurring real-time events
4. Properly handling the signals & real-time events in parallel will require communication among ranks at sync points after a signal is received - instead of adding additional allreduces to the sync object, we will unify the new checks with the existing checks for end-of-simulation/skip intervals and do them in a single operation on a custom data structure

Test plan: Add a new test suite to sst-core to test signals and real-time events