partly reverting changeset 9f7cfaec in order to correct problem in estimation: oo_.steady_state is not updated anymore in resol.m. (steady_state operator is broken in estimation).

corrected bug introduced in changeset 32c997f1 of 02/25/11 and affecting k_order_perturbation

bug correction for dr.nspred when a variable is both backward and forward (bug introduced in changeset 32c997f1 02/25/11)

fixing bug for purely backward models (introduced in changeset 32c997f1, 02/25/11)

cleaning set_state_space.m, taking into account that all modesl have leads and lags on only one period.

SWZ: Revert "SWZ: remove gsl from build system, to be reintstated if gsl remains a requirement in the ms new code"

This reverts commit e19026d1.