I'm likely way out of my depth here but thought I would mention it. I was attempting to use the crate in a compartmental biological model, and I require extracting iterative values of some non-state variables, as well as the state variables. I and another person also had the same problem. Short discussion here: https://users.rust-lang.org/t/extracting-non-state-variables-from-ode-solver/137823.
We might just be missing something simple, or perhaps the structure of the crate requires modification? For my applications I would require this, and the ability to handle ~20-40 compartments.
Let me know if I can help. Thx. J.