Relate purchase order with invoice

This module establish a relationship between purchase order and the invoice, from an already generated invoice perspective. It checks if the invoice matches the purchase order but without adding the purchase order lines to the invoice. This module changes the original behaviour of Odoo, since it prevents to replicate the lines in the invoice and Allows the operator to have several criterias for approving or rejecting the invoice if it is generated from an external source, or if it is generated from account module without following the natural purchase workflow in Odoo.