2nd solution to #734

void
MSet::Internal::set_new_weights(vector<double> weights) {
    for (int i=0;i<weights.size();++i) {
        Xapian::Mset::Internal temp(weights[i],items[i].did,items[i].collapse_key,items[i].collapse_count);
        temp.sort_key = items[i].sort_key;
        items[i].swap(temp);
    }
std::sort(items.begin(),items.end(),comparator); //comparator compares the weight of the items.
}

template <typename Iterator>
void
MSet::set_new_weights(Iterator first, Iterator last) {
   internal->set_new_weights(vector<double> (first,last));
}

void
Ranker::rank(Xapian::MSet & mset, const string & model_key, const Xapian::FeatureList & flist)
{
    LOGCALL(API, std::vector<Xapian::docid>, "Ranker::rank", mset | model_key | flist);
    std::vector<FeatureVector> fvv = flist.create_feature_vectors(mset, letor_query, Xapian::Database(db_path));
    load_model_from_metadata(model_key);
    std::vector<FeatureVector> rankedfvv = rank_fvv(fvv);/*Not sorted the FeatureVectors are in the original order of the  documents*/

    std::vector<double> weights;
    for (size_t i = 0; i < rankedfvv.size(); ++i) {
        weights.push_back(rankedfvv[i].get_score());
    }

    mset.set_new_weights(weights.begin(),weights.end());
}