Teach the AI to attack barbarian camps

[QuintillusCFC]

Playing through 150+ turns, it became clear that the AI should learn how to clear out barbarian camps.

With random exploration in Babylon, the AI would de facto clear out camps because enough random movement around their home area eventually cleared out the barb camps.

Now that the AI explores to reveal tiles, and then retreats to defend cities, barbarian camps can remain the entire game.

This will also be a case where priorities have to be balanced - do we send out another explorer, or clear that barb camp?