Potentially make PosteriorIRF obey irf_shocks
irf_shocks option of estimation restricts the generation of IRFs in
PosteriorIRF_core1, but has no effect on
PosteriorIRF, which always iterate over
M_.exo_nbr. While this does not affect the number of plots as only IRFs>10^(-6) are plotted and they were initialized to 0, there may be a computational gain in not computing posterior IRF moments for those shocks.