[ENH] Do some residual analysis to ground the spatial model search

[ljwolf]

This will be done in the spatial regression book chapter, and should be ported to the next time these notebooks are used.

1. look at the residual map of aspatial regression
2. look at a regional model of regression residuals (e.g. make boxplots of residuals by region)

Again, if we synthesize this from other in-development material, this will be provided.