Fix multiple first-level headers in an R markdown file

Bookdown markdown headers are special in that each chapter *must* start with a first-level header `#`.

fixMultipleFirstLevelHeaders(rmdfile)

Arguments

rmdfile	character(1) path to a `.Rmd` file

Details

A common workflow is to have each "chapter" respresented as a single `.Rmd` file. Functions to check and then fix `.Rmd` files programmatically facilitate finding and fixing vignettes that do not meet these criteria.