`constraint_connection_unitary_gas_flow` implementation doesn't hold for node groups

[mihlema]

This constraint is implemented to ensure that gas will only flow in one direction; which works fine for gas flows between two nodes. However, if we were to use this constraint in the future, for multiple commodities through the same pipeline, we might have parts of the flow going one direction, while others flow in the opposite direction. This would require defining the binary_gas_connection_flow variable on the node_groups (representing the mix of different commodity flows), using the highest resolution of this node_group.

[DillonJ]

Possibly something we want to include in milestone TB 1.0?

[clizbe]

@mihlema Is this still relevant? Should we revive this issue? @datejada Maybe TNO can take a look at the formulation?