1.38 candidate 1 - not really a release

- began rewrite of compare() to default to DIC; need clever way to
allow WAIC as option
- new class definition for map, adding slots formula_parsed (will be
used internally by plp plots NYI) and links (used by link)
- link supports map and map2stan now
- updates to man pages, mainly converting old linear model examples to
show <- instead of ~
- placeholder for plp plots, which will be prior-likelihood-posterior
plots of model fits