Even smarter smart-barriers

[rogerpearce]

I think we should add support for a post-barrier registered call back in comm. This would allow any registered class to be notified right after a barrier has completed.

On a similar topic, I've been thinking it might be a nice feature to support a RAII style barrier to identify "asynchronous regions" and potentially name them for logging purposes. For example:

  {
     auto region = ygm::make_async_region(comm, "My Very Important Calculation");
     // Could add support for optional logging in region
     
     //
     //   lots of async operations
     //
   
     // region goes out of scope, causes barrier, optionally dumps logging in post-barrier
  }

Edit: nested regions could be supported.