Add support for transitions between formations smaller than the fleet size

[ntamas]

Steps to reproduce:

1. Create a storyboard where the first formation after the takeoff has a smaller number of drone slots than the entire fleet size.
2. Plan a transition to this formation.
3. Create a new formation that would need the entire fleet.
4. Add this formation to the end of the storyboard.

Observed result:

The transition planner fails because it only tries to use the drones that already participated in the last formation of the storyboard (which was smaller than the fleet size) and did not try to use the ones that were "left out" from that formation.

Expected result:

A transition should have been planned to the new formation using all the drones.