Strategic AI: Threat evaluation, taking pathing into account

Continuation of Strategy AI implementation.

When AI evaluates threat, it will now consider how far a group can move, so that area directly within the reach of an enemy group will be considered to be same level of threat. This also takes obstacles into account.

When evaluating threat, friendly groups will exert a marginal threat reduction. Meaning that it's safer when you're close to your friends.

All of the threat sources overlap, making it so that two enemies that are close together project more threat than alone.

Threat values are normalized, so other groups project threat proportional to their relative strength vs that of the AI.