Make it possible to assign priorities to locations to be used for stock.move assigning

Currently stock.move.check_assign() assigns locations to moves in a non-defined order (usually lower id first). This is a problem if the locations don't have the same accessibility in the warehouse, for example some of them are overflow locations on higher shelves, the others are primary pick locations.
To be able to control the location assignment logic, the locations' posx, y, z fields could be used to define an order.

There is a possible implementation in the related branch modifying the sql query in stock.location._product_reserve().