[REF] Make modules really modular

product_get_cost_field and product_cost_incl_bom Tried to be modular but failed because of the limitations of the API in v7.
This works around those limitations by injecting the required changes directly into the parent Model.
[ADD] product_cost_purchase

Replenishment cost based on the latest purchase order/quotation, for goods with very ample and frequent price changes (stock exchange)