new method of extracting the most similar subset pairs

This approach has a couple of important differences, and a bug fix.

FIrst, we stick with condensed arrays, which is neater and more
efficient.

Second, I realised there's a slight (but unimportant) bug in the way it
worked previously. Previously I would group all subset pairs that were
equidistant, but that might have ended up grouping two radically
different pairs of subsets, as long as within each of those pairs the
distance was the same. In practice this bug would almost never occur,
becuase we almost never see equdistant subsets.

The new approach only groups pairs. This is neat, because now
--rcluster-percent 100 should be identical to the greedy algorithm. And
--rcluster-percent 100 --rcluster-max 1 should be identical to the
hcluster algorithm. Thus, the new rcluster algorithm really does
generalise the other two approaches.

Neat.
Better user output.