This fixes#93.
- filter invoices that are already in draft and open payment orders.
- use residual amount to care for partial reconciliations and align with official 8.0
We drop the bank statement related part and we manage
the state of payment order with workflow transitions
triggered by the reconciliation of moves lines on the
transfer account.
Since the field is required and payment modes may exist when
account_banking_payment_export is installed, a default value
is necessary to let Odoo set the required database constraint.
