Turned penalized_bscore tagged_item as std::pair

tagged_item are good when you need set some order on the item which
differs from the default item order. We don't need to sort
penalized_bscore by there complexity so I don't see why we need
tagged_item.

In addition changing it to std::pair has revealed some minor coding
error due to the fact that tagged_item can automatically be converted
into the item (I really don't understand how though!).