Adding account vat on payment 7

Moved from https://code.launchpad.net/~domsense/account-payment/adding_account_vat_on_payment_7/+merge/196914
The purpose of the check ```if vat_on_p and vat_on_p != valid_lines``` is to guarantee that, if the voucher contains at least 1 'vat on payment' invoice, then every invoice of the voucher must be 'vat on payment'