[FIX] account: no payment terms on refund

In the case where:
create an invoice with payment terms that contains at least 2 term lines
Validate
create a refund by clicking cancel and reconcile

Before this commit, the refund move lines were wrongly created and reconciled with the invoice's move lines

After this commit, since payment terms are irrelevant on refunds, we set it to false and the problem doesn't raise

OPW 804752

addons/account/models/account_invoice.py

@@ -985,6 +985,7 @@ class AccountInvoice(models.Model):
         values['state'] = 'draft'
         values['number'] = False
         values['origin'] = invoice.number
+        values['payment_term_id'] = False
 
         if date:
             values['date'] = date
@@ -1365,6 +1366,7 @@ class AccountPaymentTerm(models.Model):
     def compute(self, value, date_ref=False):
         date_ref = date_ref or fields.Date.today()
         amount = value
+        sign = value < 0 and -1 or 1
         result = []
         if self.env.context.get('currency_id'):
             currency = self.env['res.currency'].browse(self.env.context['currency_id'])
@@ -1373,7 +1375,7 @@ class AccountPaymentTerm(models.Model):
         prec = currency.decimal_places
         for line in self.line_ids:
             if line.value == 'fixed':
-                amt = round(line.value_amount, prec)
+                amt = sign * round(line.value_amount, prec)
             elif line.value == 'percent':
                 amt = round(value * (line.value_amount / 100.0), prec)
             elif line.value == 'balance':