docs: def extant_item(arg, arg_type):
      """Determine if parser argument is an existing file or directory.
  
      This technique comes from http://stackoverflow.com/a/11541450/95592
      and from http://stackoverflow.com/a/11541495/95592
  
      Args:
          arg: parser argument containing filename to be checked
          arg_type: string of either "file" or "directory"
  
      Returns:
          If the file exists, return the filename or directory.
  
      Raises:
          If the file does not exist, raise a parser error.
      """
      if arg_type == "file":
          if not os.path.isfile(arg):
              raise argparse.ArgumentError(
                  None,
                  "The file {arg} does not exist.".format(arg=arg))
          else:
              # File exists so return the filename
              return arg
      elif arg_type == "directory":
          if not os.path.isdir(arg):
              raise argparse.ArgumentError(
                  None,
                  "The directory {arg} does not exist.".format(arg=arg))
          else:
              # Directory exists so return the directory name
              return arg