Interweave internal_sources and collect_demand allocation runs

[SouthEndMusic]

Since #1429 we set the source capacities to 0 when collecting subnetwork demands, to make sure that only flow from the main network inlet(s) is used to collect subnetwork demands. However, user return flow and buffers can create internal sources for later priorities, making the subnetwork demand not fully constrained once again. We could go the route of setting the source capacities to 0 before each priority, instead of only before the whole collect_demands run as we do now. We could also invest more time into giving priorities to sources (#565), which is a more generic approach which also makes better use of local available water.

[SouthEndMusic]

This was discussed by @SnippenE, but more in detail by @gijsber and me. We came to the conclusion to no longer separate the internal_sources and collect_demands runs, but to interweave them. That is: for every priority, first do an internal_sources and then a collect_demands run. That way internal sources that are created during demand collection are always used up before the next collect_demands run, and their capacity does not have to be thrown away.
But I still think the internal source capacities should be temporarily set to 0 when collecting subnetwork demands to be absolutely sure that flow can only come from the main network.