1.54 - Titanic Iceberg

- bugfix for map2stan imputation combined with certain kinds of varying
effects. Added a test for this.
- man pages for pairs.map2stan and plot.map2stan
- plot.map2stan (trace plots) now recognize n_cols and max_rows
arguments for arranging plot grid
- WAIC method for map2stan defaults to use all non-warmup samples
returned by Stan