move fetch from init to have_restart

[adammoody]

This moves the fetch operation from SCR_Init to SCR_Have_restart. Datasets are still rebuilt during SCR_Init, and flushed if SCR_GLOBAL_RESTART is set.

This allows a user to specify the checkpoint that SCR should load by calling SCR_Current before SCR_Have_restart. Currently, one must specify the checkpoint name before calling SCR_Init, but some applications would prefer to call SCR_Init before they have identified the name of the checkpoint they want to load. In particular, for applications that scan the parallel file system for checkpoints, this enables SCR to rebuild and flush any cached checkpoints during SCR_Init to make those checkpoints visible to the scan. The application can then call SCR_Current to provide the name and before calling SCR_Have_restart.

Resolves #475

This PR also enables a user to call the Restart API after having written a dataset with the Output API. This is useful for some applications that test checkpoint/restart by writing a checkpoint and then immediately reading it back to restart.

Resolves #392

This PR also modifies the behavior of SCR_FETCH=0. The previous behavior disabled SCR from loading a checkpoint from the prefix directory. Now this setting causes SCR to switch into bypass mode during restart so that SCR_Route_file directs the application to read its files directly from the prefix directory.