Add uncertainty for the traditional (non-Bayesian) models

[drbenvincent]

Suggested by @juanitorduz. Would be good to get measures of uncertainty for the non-Bayesian models. Could use:

• https://github.com/scikit-learn-contrib/MAPIE
• https://github.com/statsmodels/statsmodels

Statsmodels notes

• Using R-style formulas https://www.statsmodels.org/dev/example_formulas.html (also see alternate constructor)
• Out of sample prediction: https://www.statsmodels.org/dev/examples/notebooks/generated/predict.html

[NathanielF]

I like statsmodels but we should also consider linearmodels as per @juanitorduz 's suggestion.

I can investigate both backends for the frequentist IV ticket and present the workflow with both for comparison if that helps

[drbenvincent]

FYI: We are pushing ahead with the Bayesian causal inference methods. So there are no immediate plans for this. But if someone out there wants to give this a go, then feel free. It is also worth reviewing the Discussion here #250