def download(remote_file, local_file)
      raise ConnectionClosed.new('Connection is closed.') unless @ssh
      sftp.download!(remote_file, local_file)
    end
def download(remote_file, local_file)
      raise ConnectionClosed.new('Connection is closed.') unless @ssh
      sftp.download!(remote_file, local_file)
    end
Uses SFTP to download a single file from the host.