[IMP] automatic fiscal positions for simple cases

Add group of countries res.country.group
Add get_fiscal_position method a method to compute a fiscal position based on company_id, partner_id, delivery_id
The meaning of res.partner.fiscal_position is now a forced a fiscal position.
The default implementation should handle simple cases, like VAT in UE and sales
tax in the US, but the method can be overriden to handle more complex ficals
rules.
[FIX] purchase_requisition test of PO's