t is a matrix describing the aggregation.
     Each aggregation step entry produces a row, listed in the order they
     were performed.  The first two columns are the indices of the clusters
     that were aggregated; the index assigned to the aggregate for future
     entries is the lower of the two original indices.  The third column is
     the connection strength between the clusters before aggregation and the
     fourth column is the isi statistic for the aggregate (0 if isi statistics
     are not b