Purchase Request Module
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The Purchase Request module introduces the concept of Purchase Requests which
are central points to collect purchase requests generated by other process from
Tryton.

Purchase Request
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A purchase request defines the need to purchase a product. It
is linked to a warehouse and possibly a supplier, it contains a
ideal purchase date and an expected supply date. A purchase request
can be in one of these states:

* Draft

  A draft purchase request is a request that is not linked to a
  purchase.

* Purchased

  A Purchased request is a request that is linked to a purchase which
  is not in state Done or Cancel.

* Done

  A purchase request is in state Done if it is linked to a Purchase
  which is in state Done.

* Cancel

  A purchase request is in state Cancel if it is linked to a Purchase
  which is cancelled.

A wizard allows to create purchases based on a selection of draft purchase
requests. The new purchases contains one purchase line by product and unit and
group them by warehouses and by suppliers. Once the purchases are created, the
corresponding purchase requests are set to the state Purchased.
