More efficient code using recursive approach to compute the mean and covariance matrix.

The posterior mode, obtained from the metropolis-hastings, and the inverse of the estimated covariance matrix are saved in <NAME_OF_THE_MOD_FILE>_mh_mode.mat.

git-svn-id: https://www.dynare.org/svn/dynare/dynare_v4@1285 ac1d8469-bf42-47a9-8791-bf33cf982152