Persistent Checkpoint Prototype

[mattdailis]

• [x] Demonstrate serializable cells
• [x] Demonstrate restarting activities
• [x] Demonstrate restarting subtasks
• [ ] Demonstrate poking in new values
• [ ] Demonstrate conditionally omitting activities from being restarted
• [ ] Consider what to do if sim config affects what cells are allocated - can we either be robust to this, or forbid it?
• [ ] Investigate approaches to minimizing size - maybe gzip?