[EM] Filling the last PR-STV seat using a condorcet method

[Raph Frank]

This would basically be a change to the last round for PR-STV.

Once (N-1) seats have been filled, the last seat is picked using a
condorcet method.  All ballots would still have their adjusted
weighting, so a ballot that was used to elect one of the already
elected candidates would be given a lower weight.

This breaks later no-harm for filling the last seat, but potentially
increases the quality.  Ordinarily, the last seat is completely
elected via IRV, and has the issues with that method.

Since that is the case, it may be worth uneliminating all the
candidates.  However, that means that a vote could be deweighted
because it was used to elect a candidate, even though it could have
helped elect a preferred candidate in the last round.

This method collapses to condorcet in the single seat case, but that
is somewhat cheating.