Make custom trapezoidal integration callback analogous to IntegratingCallback

[SouthEndMusic]

As explained in #1444, the approach for integrating flows proposed in #1431 is not feasible. The next idea was to use integrating callbacks now that these have more support. However, the integration scheme used in those callbacks is not feasible for us because we cannot evaluate the values to be integrated in intermediate points used for Gaussian quadrature on which the integrator doesn't stop.

Therefore the current idea is to make our own analog of IntegratingSumCallback (see the link above) with the trapezoidal integration we already used, but now more aligned with the DiffEqCallbacks.jl approach.