Given complete dataset objects in Neotoma (must have used
get_datasets() or get_downloads()), return the dataset
DOI for the record.
# S4 method for class 'sites'
doi(x)data.frame object with DOIs information.
if (FALSE) { # \dontrun{
# Get datasets metadata from API and retrieve DOIs
ds <- get_datasets(1)
doi(ds)
} # }