Allocation main network initialization

[SouthEndMusic]

Fixes #921.

[SouthEndMusic]

After doing a manual depth-first traversal of the call tree sprouting from function AllocationModel, I found the need for these modifications to support the main network:

• In function allocation_graph_used_nodes!: Make sure that for each edge that connects the main network to a subnetwork, both nodes connected to this edge are used in the main network problem (which indeed means that the optimization problem for the main network considers some nodes which are not in the main network)
• In function add_variables_absolute_value!: Use the outgoing node in the main-sub connection edges as a user for the main network
• In function add_constraints_absolute_value!: Add constraints also for the nodes mentioned above.

Other functions in allocation.jl also need to be checked for whether they need updates to support the main network, but this is left for #922.