Allow users to determine the threshold below which IRFs are suppressed.

- Prepares moving future IRF options to a substructure
- Adds new option for determining threshold
- Harmonizes threshold values across estimation and stoch_simul
- Fixes some header typos
Add debugging information for not displayed IRFs due to size
Merge remote-tracking branch 'jpfeifer/irf_debug_feedback'
Preprocessor interface for irf_plot_threshold

Closes #498