Creating the integrand for `solve_ivp` in Fortran

The problem

Our solve_ivp function expects an integrand. We can't pass integrands from Python, so we need a way to essentially make them on the fly in Fortran. However, Fortran doesn't support higher-order functions. I'm not sure what exactly the right pattern for such behaviour is.

Definition of "done"

• We have a pattern for creating our integrands within Fortran so that we can pass them to solve_ivp, the way this is currently set up this will require something like a higher-order function but maybe there are other solutions

Additional context

https://gitlab.com/magicc/fgen/-/blob/c98fa9a5cd1e9f6d66a3104e2291bbc74a47097f/tests/test-data/two_layer_model/two_layer_model_solver.f90#L1 shows one way to achieve behaviour like a higher-order function (although it feels like a hack).