We're trying to board units onto our ships.
We're moving ships and eventually unload units.
add a unit to this plan
TODO not currently used ... to be fixed
count available slots
when avoidEnnemy is true, we try to not board/unboard in ennemy territory
tell if a unit is garrisoned in one of the ships of this plan, and update its metadata if yes
remove a unit from this plan, if not yet on board
Try to move on and then clear the plan.