Negative prices and margins

License	yes
Release version	Core: 10.17.0, API: 6.1.0

This functionality enables Prodrisk to run with negative prices and, in the case of production cost/usage cost, negative margins.

This has previously not been possible in the model due to incorrect ordering of PQ segments. Each weekly optimization problem in Prodrisk is linear, meaning that no binary decisions can be made to order the PQ segments for a specific plant. This is fine whenever the margin is positive since the PQ curves are convex and it will be optimal for the model to use the segment with the best efficiency first, leading to correct ordering. When the margin is negative, however, it is optimal for the model to use the least efficient segment first. The PQ curve is therefore loaded incorrectly, leading to unphysical behavior.

The new functionality generates a 1-segment PQ-curve between \((0,0)\) and \((Q_{\textup{max}}, P_{\textup{max}})\) for a specific plant whenever the margin is negative. The original PQ curve is kept in time steps when the margin is non-negative. The linearization of the PQ curve is illustrated in the figure below.