Save all results via saving callback (follow up)

[SouthEndMusic]

This is a follow up of #1825 (and a few unrelated experiments).

Fixes #1837.

Other changes in this PR:

• Storages from the cumulative flows are computed as a sparse matrix-vector product
• Storage rates are computed without involving the initial storage
• Manning resistance is more stable around $\Delta h = 0$.

[visr]

This looks good to me. If you could add some unit tests for flow_to_storage, this can go in.

[evetion]

• Manning resistance is more stable around Δ h = 0 .

IMHO, that is an (important! 🎉) change that should not be committed by a squashed commit called save all results via saving callback.