`covariance_diag_store` Creates a list storage of approximate diagonal covariances computed by the function diagonal_covariance_i
covariance_diag_store(f_data, K)
A list containing K 2-dimensional arrays containing the diagonal covariance matrices of the functional data, for lags h in the range 1:K.
the functional data matrix with observed functions in the columns
the range of lags 1:K to use