Add cross-border flows to scenarios

[tiernan-buckley-ufr]

Scheduled cross-border commercial exchanges are a component of the market that are yet to be implemented as part of the scenarios.

In the examples, a "cross_border_flows.csv" should be added to the inputs containing data for Germany's scheduled commercial exchanges, with exports being treated as negative and imports as positive.

A "cross_border_flows_units.csv" is to be made configuring how the cross-border flows participate in the market, where exports are treated as a form of demand and are set as a bid just below the (price-inelastic) demand bid price to ensure they are realised, and imports are treated as a form of generation set near to the min. bid price to ensure the are realised.