[IMP] purchase: make purchase report extendable
[IMP] purchase: make purchase report extendable

The idea is to do like the sale report: being
able for other modules to extend the report and
add their own fields. As we are going to split
stock from purchase, we will need this.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo