docs: def format_and_annualise(self, raw_cov_array):
          """
          Helper method which annualises the output of shrinkage calculations,
          and formats the result into a dataframe
  
          :param raw_cov_array: raw covariance matrix of daily returns
          :type raw_cov_array: np.ndarray
          :return: annualised covariance matrix
          :rtype: pd.DataFrame
          """
          assets = self.X.columns
          return (
              pd.DataFrame(raw_cov_array, index=assets, columns=assets) * self.frequency
          )