Allow automatic product weights to add in weights set on component stock items

This request has been split from Default HS/Commodity codes and weights on products - #18 by peter-burger1 as some extra ideas were added when the original project was right at the finish line.

This feature request thread is to further build on the use of container types having default weights for products, by allowing weights to also be set on component stock items and this be included in the product’s weight.

If you would find this useful, please give this thread a vote with the button in the top-left :+1: