Invalidate checkpoint files when config dict changes

[rmjarvis]

The docs have this warning currenty:

.. warning::

    Be careful to manually delete any check-point files if you have made any changes to to the configuration between runs.  Currently, *imSim* only checks if a file for a individual sensor already exists.

I think we can fix this by including a hash of the base dict in the checkpoint "name" to automatically start over when the config file changes.

[cwwalter]

This would be a good idea. I just got bitten by this in a way I was surprised by.

I was making changes to stamp.py. There were some mistakes but when I ran the program, it was fine and they didn't get caught. That's because the stamp/output was being skipped all together due to the presense of the checkpoints. It was caught by the tests in the CI. It took me quite a while to notice what was going on.