simplify loss constraints

[HaraldGSvendsen]

At present, edgeFlow is defined at the "from" end, flow=flow12-flow21.

Instead, define edgeFlow in the middle, such that flow12 is flow in positive direction out of "from" terminal, and flow21 is flow in negative direction out of "to" terminal
The loss function can use flow12+flow21 = absolute value for lookup. We don't need to split between loss12 and loss21.

This will simplify terminal energy balance equations.