Various bug fixes and small adjustments

1. Port to Occbin smoother the same computational improvements done for standard one under smother_redux option. This also require to provide occbin reduced state-space matrices as output argument of missing_DiffuseKalmanSmootherH3_Z.m
2. Filter out singular Q matrix when solving for the unconditional variance of states using compute_Pinf_Pstar.m
3. Fix bug where filtered and updated variables from Occbin smoother contained the steady state twice.
4. More robustly compute log determinant for marginal data density.
5. Remove invalid characters for plot_shock_decomposition