Optional upgrade modes for Reactors, Generator and Radiators

Uses an FNUpgradeModule to upgrade during OnStart, after TweakScale is
applied, still no retrofit.

GetInfo and Editor part GUI get some details related to upgrades, only
when upgrades are enabled.

Also: Fixed phantom CP generation on Fusion Reactor, compute Isp more
eagerly on Thermal Rocket Nozzle & Turbojet.