Utility function for generating PDF reports

[dan-knight]

This should be a generic wrapper for the rmarkdown::render() function. It will likely be a private function that can be used by other, more specific public functions to generate a particular type of report.

Report types will likely be stored in the /inst directory as .Rmd files, which can be called withrmarkdown::render() to generate a report. This should be able to pass variables to the Rmarkdown file, though this will require some research.