Thermal model updates: feasibility & unit tests

[werdnum]

Three significant changes:

1. Add an option (to become the default?) to use penalties rather than constraints for thermal conditions. This will replace a bunch of hacks I'm using to avoid trying to ask for something impossible from the solver.
2. Rework overshoot temperature - instead of "never exceed this temperature", we want "if we do exceed this temperature, it shouldn't be our fault" - implemented as "load must be off if we overshoot"
3. Added relatively comprehensive unit tests to cover the above behaviours plus more.