Discussing clustering improvement in COPD chapter

[daffidwilde]

I suppose the only "more" we could be is giving the specific numbers. Perhaps add them in a commit and we'll see how it looks? It's not vital but it could worth transparently saying exactly what the "improvement" (no matter how slight) is.

Originally posted by @drvinceknight in #84 (comment)

[daffidwilde]

Going to omit the paragraph now, and keep a notebook handy with the results in.