Letor: returning MSet after re-ranking

Hello James,

As it was discussed, at present letor_rank(const Xapian::MSet & mset)
method returns a vector of sorted docids after performing re-ranking,
whereas ideally, it should return a re-ranked MSet.

To be able to do this, I was thinking of adding rerank_mset(vector<double>
updated_weights) method to mset.h, which basically updates the weight of
each MSetItem in MSet with weights (or score, in terms of letor) obtained
after re-ranking, and then sorts the vector<Xapian::Internal::MSetItems>
items by these updated weights.

What do you think about this?

Regards,
Ayush