CAS would never be planned for turn 0 because there were no enemy ground
units, so a breakthrough attack was planned and CAS was considered
unnecessary. Fix that by rejecting all frontline stance tasks when there
are no *friendly* ground units to command.
Another issue is that CAS missions were being planned even when there
were no enemy front line forces. Skip planning in that case except on
turn 0 since we expect there to be enemy ground units on turn 1.
Fixes https://github.com/dcs-liberation/dcs_liberation/issues/1629
The doctrine/task limits were capturing a reasonable average for the
era, but it did a bad job for cases like the Harrier vs the Hornet,
which perform similar missions but have drastically different max
ranges. It also forced us into limiting CAS missions (even those flown
by long range aircraft like the A-10) to 50nm since helicopters could
commonly be fragged to them.
This should allow us to design campaigns without needing airfields to be
a max of ~50-100nm apart.
This improves the AI behavior by choosing the stances non-randomly:
* Breakthrough will be used if the base is expected to be capturable and
the coalition outnumbers the enemy by 20%.
* Elimination will be used if the coalition has at least as many units
as the enemy.
* Defensive will be used if the coalition has at least half as many
units as the enemy.
* Retreat will be used if the coalition is significantly outnumbers.
This also exposes the option to the player.
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.
