Include dumped energy as an optimisation criterion

[BenWinchester]

Issue

When running optimisations, there's nothing in the outputs (as it's not a criteria) that enables a user to examine the dumped energy as a proxy to system utilisation.

It would be nice if this was something that a user could optimise based around.

Proposal

Introduce two new threshold criteria:

• Dumped energy, which would be the total dumped energy in the iteration period;
• Utilisation fraction, which would be the fraction of PV assets which are utilised. I.E., 1 - dumped_energy_fraction.

Alternatives are doing this afterwards.