Add a $shipment->getTotalQuantity() method

Add a method to get the total amount of products in a shipment.

Currently I'm using something like this:

$order_total_quantity = null;

foreach ($shipment->getItems() as $shipment_item) {
    $shipment_item_quantity = $shipment_item->getQuantity();
    $order_total_quantity  += $shipment_item_quantity;
}