Heatmap.R: first draft implementation of summarizing many runs into a 2d picture.

This is inspired by fight-entropy.com/2012/07/visually-weighted-regression.html
but it is not the same because it's discretized, and I cannot tell if
it is or is not even theoretically equivalent, since that one is based on:
 - bootstrapping a single dataset
 - doing a regression for each bootstrap
 - running a smoother on the regressions, vertically

and this is
 - pretending each run is a different line (in the previous, a different bootstrap)
 - throwing every run into one big bucket
 - running a smoother (kernel density estimates) vertically