Dev-Freeze Engine Tweaks + Powernet Fixes

* Fixes APC load totals not being reset when an area power update is done, which sometimes resulted in double the power usage
* Fixes pressure regulators not checking for unregulated mode correctly
* Fixes freezer heat transfer being accidentally capped by power usage. This broke the engine as it meant engine temperature would not settle on a finite value. Normally increasing the engine power increases the heat output. As temperature goes up, the heat removed by the freezers also goes up, until the temperature settles at some value. This bug disrupted this process, causing temperature to go up indefinitely above a certain power output.
* Updates and (attempts to) rebalance SM for dev-freeze. On a default setup (map changes will be added soon), tried to make it so that the SM can run up to 500 kW at ~ 700 K. Spent lots and lots of time playing with the engine locally.