Performance regression

[BenSchZA]

In radCAD version 0.11.0 (see CHANGELOG.md) an update was made to ensure that unintended state updates between state update functions within a single partial state update block can not be made. As a result, some models may perform worse - typically models with lots of states and state updates or with large objects in state.

There is a compromise to be made between optimising performance vs. ensuring immutability - TBC what the best solution is. It's likely that it should be handled on a case by case basis. For example, see radCAD deepcopy and deepcopy_method options.

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