airbase sensor: limit number of active planes on the airbase

[jtoppins]

This is especially important for carriers.

Notional limits:
ships: 1
helo carriers: 2-3
plane carriers: 4-8
airbases: 8-12

Events to track number of active planes on the airbase:

• land
• takeoff
• birth
  • this event will need to be fixed up by the Theater class
  • set place to closest airbase if the unit is an aircraft or helo and not in air before forwarding to any airbase agents