Make PosteriorIRF obey irf_shocks

The matrices are still initialized to have the size M_.exo_nbr, but the computations (e.g. moments) are now only performed for the shocks in irf_shocks. PosteriorIRF_core2 still does not allow for a selection, but the check for IRFs>10^(-6) already effectively prevents the plotting.
Merge pull request #403 from JohannesPfeifer/irf_shocks

Make PosteriorIRF obey irf_shocks