Run allocation at t = 0

[visr]

Currently allocation doesn't run at t = 0. This is done because flow on the source edges is still unknown. This means that effectively there is a free-for-all situation for the allocation interval. It would be better to special case t = 0, to run the RHS once to get a first estimate for the instantaneous flows at t = 0, and then run allocation with that.

[SouthEndMusic]

With #1427 it is required to set these initial instantaneous flows* as the mean flows.

.* To be precise, set the instantaneous flows times the allocation interval as the mean flows, as there is a division by the allocation interval at the start of update_allocation!.