Save allocation flows

[SouthEndMusic]

Fixes #967.

Blocking for merge: #1006.

Note to @visr and others working on flow visualization in QGIS: although this yields a flow output file, it is different from the flow output file for several reasons:

• There are demand gathering flows and allocation flows (which yields 2 flow values for each edge in a subnetwork for each allocation timestep)
• (For now) an edge connecting the main network to the subnetwork yields flows from both the main network and the subnetwork.

[SouthEndMusic]

To do:

• Now flows are saved for allocation flow edges, these have to be translated to edges from the physical layer;

After merging #1006:

• add the gathering_demands column;
• move p.allocation_record to allocation.record.