Allow forecasting with exogenous variables

[jessegrabowski]

Enable scenario-based forcasting in VARMAX, SARIMAX, and structural models with regressions.

Couple to-dos before its ready for review:

• [x] Carefully check the timing on the forecasting. Things seem to be shifted forward by 1 or 2 periods.
• [ ] Recycle the exogenous forecasting code to allow in-sample counterfactual scenarios using sample_conditional_posterior or sample_conditional_prior.
• [ ] Update relevant example notebooks to show how to use the scenario tools

Otherwise things seem to be working