Add Modifiers for levies including terrain boni

[dav77-b]

Add Modifiers for levies

And if it was only for modding support:

levy_damage_mult
levy_damage_add
levy_toughness_mult
levy_toughness_add
levy_screen_mult
levy_screen_add
levy_pursuit_mult
levy_pursuit_add

and terrain_boni

Make a field for them so we can mod them:

levy = {
type = levy

damage = 10
toughness = 10
pursuit = 0
screen = 0

terrain_bonus = {}

low_maintenance_cost = { gold = skirmisher_low_maint_cost }
high_maintenance_cost = { gold = skirmisher_high_maint_cost }

mercenary_fallback = yes
holy_order_fallback = yes

icon = levy
}