Main feats:
+ Create purchase order when confirm the requisition
+ Send mail when notify_purchase has been clicked
+ Only user in purchase group can convert requisition to purchase
order
+ Every item in requisition line must be set a supplier before
converting to purchase order
+ Read-only when requisition is done or cancel
+ I don't limit min qty from supplier info, if it is needed I will
add it in future
Notice:
+ You should update purchase module due to purchase.order has
been inherited in requisition module
+ Mail function has not been tested yet