corrected bug in computation of dr.ghs2 introduced in commit 84eaaaf7

cleaned code that is useless since we transform leads and lags on
period > 1

Ref. manual: list in a single place the MATLAB toolboxes and Octave-Forge packages from which Dynare can benefit

Closes: #224

is contradictory with diffuse_filter or unit_root_variables
declaration. Models with non-stationary variables, but only stationary
observed variables need diffuse_filter option and make a useless call
to kalman_filter_d (this seems better than trying to distinguish these
rare cases)

are not strictly positive.

'steps' values)