def install
      fail 'Must specify an install path.' unless install_path
      FileUtils.mkdir_p install_path
      repo.extract install_path, reference: reference
      self
    end
def install
      fail 'Must specify an install path.' unless install_path
      FileUtils.mkdir_p install_path
      repo.extract install_path, reference: reference
      self
    end
Copy the files to the {#install_path}.
 @return [External] this external object