Fix plotting first layer instead of failing by default for heterogeneous RC-L

Commit 0702c58 was incorrect, a warning was printed for all models with only
one layer.
Add col.ellipses argument to plot.assoc() functions

Initial work by Michael Friendly.
Support plotting confidence bars for 1D in plot.assoc()
Fix handling of 'what' with 1D plots in plot.assoc()

This was never tested before.
Avoid plotting confidence bars outside of box in plot.assoc()

Ideally dotchart() would clip the plot region, but...
Fix wrong reference to table in Wong (2010)

Spotted by Michael Friendly.