Structure: 
	   1. call CkStartCheckpoint() at intervals or when wall clock runs out.
	   2. restart() entry will detect if we're really restarting or not
	    by using a flag set in the Migration constructor for main and
	    then call doSimulation() to continue.  If we are actually restarting
	    then restart() will call initialForces() which then proceeds to
	    doSimulation().
	    
	  

Issues:
Does Readonly data get automatically checkpointed? Yes!
What about the proxies that are stored read-only?  They get automatically
restored.

All groups and node groups need to get migration constructors since they
automatically get dumped.