Changed the rules for how helicopters behave. A bit quirky, but perhaps will satiate those dying for more "realism" out of their helicopters.

Helicopters now lose 25 HP per turn (aka 1/4 of their health) if they are not in friendly territory in order to simulate fuel, and this loss occurs even on land (this is the fault of too general XML tags). They also do not normally regain any health when fortifying outside of friendly territory. To help counteract this for longer ocean voyages, carriers have been granted a passive ability to heal adjacent units for 35 HP per turn automatically (35 - 25 for helicopters since the damage never goes away outside friendly territory), though the serial priority of the healing compared to the helicopter damage can cause helicopters to end up still somewhat damaged even when adjacent. (Healing will not occur whenever the helicopter has used some moves, so remaining stationary next to a carrier at times is important as well.) Helicopters can also fortify to slowly heal / "refuel" when adjacent to a carrier and over water (but not when on land).

Carriers' healing ability would probably work on naval units too, which is unintentional but again the fault of too general XML tags. Fotunately their healing does not work on anything on land terrain, including helicopters.

Submarines' damage against helicopters / hover units has been dramatically reduced because it's kind of dumb if the game allows you (or your enemies) to "shoot down" helicopters with submarine torpedoes.

Removed dependency on Gods & Kings since user testing suggests that it works even if you only have Brave New World.