Get rid of specifying sources in allocation by edges

[SouthEndMusic]

Currently, if you want to specify a source for the allocation algorithm you have to indicate this in the Edge table, were you have to specify the subnetwork ID of the subnetwork in which the edge should be a source. This is not intuitive, as it might seem like you have to assign each edge to a subnetwork in much the same way you have to do that for nodes. It is not clear from the table structure itself that this has anything to do with sources.

I can find #932 with a suggestion to remove this way of indicating sources, but then it was decided to keep it because it would be helpful in specifying an order of preference over sources. That is actually what this issue comes out of; discussions on how to specify source preference in the input (see also #1505).

As discussed with @visr, the nicest way to indicate a source order is to have a new table, say SourceOrder, with columns subnetwork_id (int32) and node_id (int32), where per subnetwork the source order is given by the order in this table.

Then what's needed for this issue is the same as in #932, namely automatically infer sources in subnetworks. I.e.: if a LevelBounary or FlowBoundary node is part of a subnetwork, it is a source for that subnetwork, and each connected edge is treated as a source in the allocation algorithm. Edges being sources in the allocation algorithm should be something internal, not exposed to the modeller.

Note that this would involve a breaking change, as the subnetwork_id column wil be removed from the Node table.

[SouthEndMusic]

As an alternative to removing the subnetwork_id column from the Node table now, we could give it a deprecation warning when data is added to it, and remove it in a new major release.