clarified time complexity of ensemble kalman filter in sec 8.5.4

[murphyk]

clarified time complexity of ensemble kalman filter in sec 8.5.4 to distinguish diagonal and non-diagonal F (dynamics matrix):

In summary,  for sparse (e.g., diagonal) $\vF$,
EnKF is  $O(\nhidden \nsamples + \nhidden \nsamples \nobs)$
and EKF is $O(\nhidden^2 + \nhidden^2 \nobs)$,
whereas for dense $\vF$,
EnKF is  $O(\nhidden^2 \nsamples + \nhidden \nsamples \nobs)$
and EKF is $O(\nhidden^3 + \nhidden^2 \nobs)$.