Consider hiding the tempdir() setup in examples

[cboettig]

Wrapping examples the way @coatless suggests here might make the documentation more readable to new users. Otherwise users might erroneously think they need to use a tempdir as well.

#' \dontshow{.old_wd <- setwd(tempdir())}
#' # Your code here
#' \dontshow{setwd(.old_wd)}