Improve reinforcement system

[Kexanone]

• [ ] Getter for figuring out to which target a group is assigned to
• [ ] Add group roles: "defend", "patrol", "attack".
• [ ] Despawn and redeploy units where they are needed (especially during defense) (#11)
• [ ] Make priority system modular
• [ ] Select several targets rather than one depending on the priorities.
• [ ] Move already spawned units to a new location if their assigned target is dead.
• [ ] Probability to attack FOB