Record distance value when linking hierarchical clusters

[tayden]

It'd be a nice improvement if there was a way to access the distance between the left and right subtrees of the resulting hierarchical clustering tree without having to re-run the distance function. Perhaps this information could be recorded in the canonical object. This would make it easier (and cheaper) to generate an arbitrary number of clusters from the tree instead of just decending the tree to some depth to generate 2^depth clusters.

It would be even better if there was a function bound to the result of hcluster that accepted some int number of clusters and output the clusters in a list.

[tayden]

Closing because this is the same request as #10