Add checkpoint option during simulation

[imml3457]

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Have the ability to save the entire state of the simulator at x timestep.

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Have a flag that will enable the simulator to dump the state of the simulator to file. This will enable the user to start the simulation from a specific file.

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Useful for debugging and longer runs. Will enable the user to maintain state in case of power loss/unexpected issues.

[kenblu24]

Proposal:

Add functionality to simulate() to allow for checkpointing.

• If the user does not add any checkpointing arguments to simulate(), then the behavior should be the same as before
  (backwards compatible)
• If the user provides a simulate(checkpoint=9), save checkpoints every 9 steps to /tmp/ or the equivalent on Windows/mac
• If the user provides a simulate(checkpoint=9, checkpoint_path='model_checkpoints/') folder path, then save them to that folder instead of /tmp/ etc.
• If the user provides a simulate(checpoint=9, checkpoint_callback=func), then func will be called every 9 steps to see inside the model.