Use silhouette to choose cluster number for isomap

[colinallen]

topicexplorer cluster view could be more useful if clusters are chosen non-arbitrarily by silhouette

[JaimieMurdock]

I've started working on this on a notebook. Unfortunately I used the Darwin model and ran into a number of memory issues due to the large vocabulary size, and ended up down that rabbit hole.

My next real thing is to just do this with a reasonable corpus and use the functions I've already got.