Allow (S)EP with arbitrary sequence of innovations.

The third input argument of extended_path Matlab/Octave's routine is the
sequence of shocks (T*n array, where n is the number of exogenous
variables and T is the size of the sample). If the third argument is
empty, the (stochastic) extended path is run with gaussian
innovations (this corresponds to the previous behaviour).

TODO:
 - Fix the compatibility with ep.replic_nbr
 - Check the 'calibrated' mode.