Handler Registration

Registry for checkpoint handlers (CheckpointHandler).

A checkpoint handler registry is used to register a CheckpointHandler for specific a checkpointable item name and a CheckpointArgs type. When saving or restoring a checkpoint, the registry can be used to find the appropriate handler for the item and its args. The default implementation of the registry is DefaultCheckpointHandlerRegistry. See the docstring for the default implementation for an example how to use the registry.