DEV: More checkpoint API refactoring. Changed the public checkpoint API to: - `create_checkpoint(self, nb, path)` - `get_checkpoint_content(self, checkpoint_id, path)` - `rename_checkpoint(self, checkpoint_id, old_path, new_path)` - `delete_checkpoint(self, checkpoint_id, path)` - `list_checkpoints(self, path)` All paths in the above are API-style paths, and the `nb` argument to `create_checkpoint` is a dictionary suitable for passing to `nbformat.write`. The new `get_checkpoint_content` method returns an unvalidated notebook content dictionary. It is used by `ContentManager.restore_checkpoint` to load content to be written via `save`.