Add shipment cost to outgoing moves
Following https://discuss.tryton.org/t/about-include-more-cost-to-compute-margin/3313.
I propose to add a new module stock_shipment_cost
which will add the shipment cost to a new field on the outgoing moves.
Also the sale_shipment_cost
is adapted to allow to not invoice the shipment cost to the customer and in this case, the cost will be added to the shipment cost.