Add stress argument to struc, serialization

[stevetorr]

Summary

Stress is now included as an argument to the Structure object, so structures can be instantiated with the stress property.
Unit tests have been updated so that serialization / deserialization into JSON includes checking for this field.

• Code is in the standard Python style.
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• Docstrings have been added in the Sphinx docstring format.
• Type annotations are highly encouraged.
• Tests have been added for any new functionality or bug fixes.
• All existing tests pass.

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