Create new variables for types of links (replace hyperdges)

[cmalinmayor]

Current edge variables are "base" - all constraints work on these
Types of linking variables - continuation (one-to-one with base edges), merge (two edges into one node), split (two edges out of one node). Each of these can have a cost associated, and is constrainted to match the base edges, in the same way split/appear nodes are. However, unlike with nodes, the base has no cost, and then you don't pay continuation costs for the base edges when you pick two edges in a split.