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:

• 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

[ricardoV94]

Perhaps show the use of the functionality in one of the NBs?

[jessegrabowski]

I agree that this needs some notebooks to show the new usage, but I will do them in a follow up PR