Automatically generate and assign power creeps

[Mirroar]

Power creeps are currently managed manually, and only respawned automatically. We should implement an algorithm to automatically spend power levels on power creeps based on desired powers. These creeps may then be spawned automatically in our best rooms to improve their yield.

We need to handle some special cases when assigning power creeps:

• Reassign when an associated room has been lost
• On reassignment, despawn if the power creep isn't dead
• Make sure only to reassign if really necessary

[Mirroar]

Power creeps are now created and upgraded automatically, though we still need logic for having power creeps that manage factories. Also, reassignment is done every time a power creep can respawn.

[Mirroar]

Factory powers are also assigned automatically by now.