Control from allocation

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Fixes #714

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@gijsber @Fati-Mon I made the model as discussed:

Currently there is no flow at all in this model, and here's why: allocation calculates the amount of flow available as the flow from the level boundary in the physical layer. However, at the beginning of the simulation there is no flow there yet because the outlet does nothing (the demand is 0 at the start). So there's an impass: the outlet doesn't flow because allocation doesn't allocate anything, allocation doesn't allocate anything because there is no flow.

Of course this is a flawed mechanism, which is again solved by allocation having a better model of the outlet.