add function knitr_fig_path

fix_fig_path <- function(pth) file.path("..", pth)

## We set the path for the figures globally below, so if we want to

## customize it for individual episodes, we can append a prefix to the

## global path. For instance, if we call knitr_fig_path("01-") in the

## first episode of the lesson, it will generate the figures in

## `fig/rmd-01-`

knitr_fig_path <- function(prefix) {

    new_path <- paste0(opts_chunk$get("fig.path"),

                      prefix)

    opts_chunk$set(fig.path = new_path)

}

## We use the rmd- prefix for the figures generated by the lssons so

## they can be easily identified and deleted by `make clean-rmd`.  The

## working directory when the lessons are generated is the root so the