Sweep and Clear 1.1

After a play-test, I've made significant changes to Sweep and Clear:

1. I've removed the trait that gives a marine one extra stamina, and added a trait which gives better chances in melee combat.
2. I've changed doors so that they need to be interacted with, instead of opening automatically. Aliens cannot open doors. Doors stay open.
3. I've changed the possible actions for unrevealed aliens to put more pressure on the marines/make things faster-paced.
4. If melee combat is a tie, the marine wins.
5. By default sections now don't have doors between them. Doors are only added in some places on the mission map.