Consider BAI missions planned this turn when determining if a control
point is still garrisioned for preventing breakthrough.
This isn't very accurate yet since the HTN isn't checking for aircraft
fulfillment yet, so it might *plan* a mission to kill the garrison, but
there's no way to know if it will be fulfilled.
Garrison groups should be preferred with the following priority:
1. Groups blocking base capture
2. Groups at bases connected to an active front line
3. Rear guard units
Previously they were being prioritized based on the distance to the
closest friendy control point, which is similar to this but an
aggressively placed carrier could throw it off.
An HTN (https://en.wikipedia.org/wiki/Hierarchical_task_network) is
similar to a decision tree, but it is able to reset to an earlier stage
if a subtask fails and tasks are able to account for the changes in
world state caused by earlier tasks.
Currently this just uses exactly the same strategy as before so we can
prove the system, but it should make it simpler to improve on task
planning.
