S3 method for the GNARfit class. The function returns the value of
$$-TN/2 \log(2 \pi) - T/2 \log(| \Sigma |) - 1/2 trace(E \Sigma^{-1} E'),$$ where \(T\) is the time length of the observations, \(N\) is the number of nodes, \(\Sigma = E E' /T\) is the estimated covariance matrix and \(E\) is the matrix of residuals.