Utility function that retrieves the location of the sample OPUS binary file on disk.
opus_file()
a character vector storing the location of the sample OPUS file
fn <- opus_file()
fn
#> [1] "/private/var/folders/7g/cnfrzbd555q9lmpsr1byngs80000gn/T/RtmpIKQul6/temp_libpath18124d941235/opusreader2/extdata/test_data/test_spectra.0"