Use average flows over allocation intervals for realized in allocation.arrow

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What
Use average flows over allocation intervals for realized in allocation.arrow. In the case of level demand, use the difference in storage between the beginning and the end of the basin.

How
Integrate flows in integrate_flows! for the flows that are relevant for this allocation input, stored somewhere in the allocation data structure (make sure that this goes well in subnetworks where all optimizations are run twice). For the storage difference: use the current u and the previous one which can be taken from the integrator object (which is available in the update_allocation! function).