OrderGroup.setOrders is not a setter

The implementation of setOrders is slightly odd:

```   
/**
     * Sets the orders
     *
     * @param orders the orders to set
     */
    public void setOrders(List<Order> orders) {
        for (Order order : orders) {
            addOrder(order);
        }
    }

```
This means that a set will actually add to any existing orders. The fix is trivial, but I don't know if anyone will be relying on the existing behaviou