Re #180: Add multiple mutex groups * Add multiple mutex groups to UnitEquipmentItem * Add new attribute to Race schema * Make everywhere handle multiple groups instead of assuming one * Make factory load new attribute then fall back to old (deprecated) attribute * Add method to Unit to get the blocking items (useful for future UI work to say "replace X and Y with Z") * Add IsMutuallyExclusive method to UnitEquipmentItem to determine whether mutex groups overlap