master purchase line base price hook

With these hooks, modules like _purchase_discount_ (https://github.com/OCA/purchase-workflow/tree/master/purchase_discount) don't have to make dirty patches to extend functionality, and also other possibilities, like 3x2 offers, can be made in a clean way.

Made also this PR for 8.0: https://github.com/odoo/odoo/pull/1468