'Adds item quantities together, accounting for different pack
quantities.
Returned quantity according to pack_quantity_1
Used by controllers/inv.py & modules/s3db/inv.py'

@staticmethod
def supply_item_add(quantity_1, pack_quantity_1, quantity_2, pack_quantity_2):

    if (pack_quantity_1 == pack_quantity_2):
        quantity = (quantity_1 + quantity_2)
    else:
        quantity = (((quantity_1 * pack_quantity_1) + (quantity_2 * pack_quantity_2)) / pack_quantity_1)
    return quantity
