Modifying the workflow. Everything explained in workflow/account_invoice.xml. Add error function when reversal

Override the invoice_cancel activity and add a function call after the wizard account.move.reversal (with the amazing functionality)

so we can reverse the move associated with the invoice

Added analytic fields to the Customer Invoices view. Analytic fields use the methods defined in analytic_structure.

Otherwise we can write every analysis code on every analysis slot.