assertHE comes bundled with a number of sample files in its inst/extdata
directory. This function make them easy to access
Value
If file is NULL, returns a character vector containing the names of all
files and directories available in the package's directory (extdata).
If file specifies the name of an existing example file, returns a character
vector of length one containing the full path to that file. Stops with an
error if the specified file does not exist within the example directory.