Let the solver integrate our flows

[visr]

We compute mean or total flows in several places, and do this manually in our parameters in for instance #1300.

@SouthEndMusic suggested that perhaps it would be less code and more accurate to add those "integrated flows [m3]" as a state to our solver, such that du = flow [m3/s]. We already have two types of state, one for the Basin volume, and the PID error. We can add more.

[SouthEndMusic]

Closed in favor of #1473.