Redo selected model fits with leading rather than trailing data.

Spoiler alert:  leading data doesn't really work any better.
Unsaved changes intended for last commit.
Fix cluster.R to use k-means correctly.  It turns out you can't pass a
difference matrix; you have to pass the raw data frame.
Add clustering on trailing indicators and normalize vars before clustering.
Add some utilities for working with clustering results.
Add some scatterplots for cluster-separated regression and write up
some notes on progress to date.