add contexual field to invoice form and copy value from sales if available

sale_order_contextual/models/sale_order_contextual.py

@@ -7,3 +7,9 @@ class SaleOrderContextual(models.Model):
     contextual_note = fields.Text(
         string='Contextuele opmerking',
     )
+
+    def _prepare_invoice(self):
+        vals = super()._prepare_invoice()
+        if self.contextual_note:
+            vals['contextual_note'] = self.contextual_note
+        return vals