Implement enemy multiplier buffs in battle

[lotuuu]

Add newenemy_attack_multiplier and enemy_health_multiplier and implement them. We need these fields to act as permanent buffs for enemies during a level. This will require some thinking on how we want to handle this in our battle (a permanent and undispelable modifier? handle it when we call create_units_map with some sort of new initial_buffs field? to discuss!)