Add SaveDecisionPolicy to better encapsulate various options around choosing whether or not to perform a save at a particular step. Added a policy that allows for checkpointing as often as possible, as long as a save is not already in progress (continuous checkpointing).

Add SaveDecisionPolicy to better encapsulate various options around choosing whether or not to perform a save at a particular step. Added a policy that allows for checkpointing as often as possible, as long as a save is not already in progress (continuous checkpointing). #16075