Added Volume based shipping calculator

And some small code cleanup

You have same function in BaseShipment
https://github.com/antonioperic/Sylius/blob/d815848fab3b8b8294225824e5ce1a7f509aaa1f/src/Sylius/Component/Shipping/Model/Shipment.php#L259

There was only lapsus in $item->getShippable()->getShippingWeight();