Introduce custom TrapezoidIntegrationCallback to integrate flows and basin forcings

[SouthEndMusic]

Fixes #1473.

To do:

• Update TrapezoidIntegrandAffect.integrand_value in update_basin! for integrands that contain the basin forcings (both for flow integration and bmi integration)
• TrapezoidIntegrandAffect.integrand_value_prev can be removed, if the integration is done in this order:

function (affect!::TrapezoidIntegrationAffect)(integrator)::Nothing
    (; integrand_func!, integrand_value, cache, integral) = affect!
    (; dt) = integrator

    cache .= integrand_value
    integrand_func!(integrand_value, integrator.p)
    cache .+= integrand_value
    cache .*= 0.5 * dt

    integral .+= cache
    return nothing
end

• Test the integration around a discontinuity of the basin forcings
• Add docstrings, inline comments

[SouthEndMusic]

A drawback of the current way flows and basin forcings are computed is that a vector for all these values is allocated at each timestep, while they are averaged over the saveat intervals after the simulation. It would be nice if the IntegratingCallback would support passing the saveats to the callback constructor to predefine the integration intervals, which is nice for the output flows. Maybe worth making an upstream issue.

The IntegratingSumCallback is used for the vertical fluxes for BMI, and this is cumulative and in place. An option to resolve the above is to use IntegratingSumCallback also for the above, in combination with a SavingCallback.

[SouthEndMusic]

I just realized I made a wrong assumption about how IntegratingCallback and IntegratingSumCallback work. I thought that the integrand function was only evaluated at time points of the integrator, but it is in fact called at several different points:

https://github.com/SciML/DiffEqCallbacks.jl/blob/a35955b8480356503e3df492a9e02249cef00cd0/src/integrating.jl#L175C5-L177C55

The problem with this is that we don't know the value of the integrand at these in between points, at least not without calling water_balance! which is rather expensive.

[SouthEndMusic]

Although this was a fun dive into creating a custom callback, it didn't reach its goal, namely reducing the complexity of the integration code.