[FIX] stock_dropshipping: raise a warning in a specific invoice creation config

So far, we do not support the following combination:
- route set on 'dropship'
- PO invoicing set on 'picking'
- SO invoicing set on 'picking'

opw-634898

addons/stock_dropshipping/stock_dropshipping.py

@@ -1,6 +1,7 @@
 # coding: utf-8
 
-from openerp import models, api
+from openerp import models, api, _
+from openerp.exceptions import Warning
 
 
 class sale_order_line(models.Model):
@@ -20,3 +21,16 @@ class sale_order_line(models.Model):
                         res = True
                         break
         return res
+
+
+class purchase_order(models.Model):
+    _inherit = 'purchase.order'
+
+    @api.multi
+    def wkf_confirm_order(self):
+        for po in self:
+            if po.invoice_method == 'picking' and po.location_id.usage == 'customer':
+                for proc in po.order_line.mapped('procurement_ids'):
+                    if proc.sale_line_id.order_id.order_policy == 'picking':
+                        raise Warning(_('In the case of a dropship route, it is not possible to have an invoicing control set on "Based on incoming shipments" and a sale order with an invoice creation on "On Delivery Order"'))
+        super(purchase_order, self).wkf_confirm_order()