Python implementation of mixtape code

[tomcaputo]

Code outputs have been verified and match R output except in the following cases:

• Differences_in_Differences.ipynb: The design matrix in the Cunningham and Cornwell (2013) example is rank deficient. lm and lm_robust have a convergence issues. The problem seems to be with the fip variable. The statsmodels algorithm is more robust to rank deficiency resulting is different results
• Matching_and_Subclassification.ipynb: Python does not have an implementation of nearest neighbor matching in python. I may be possible to recreate one use standard KNN tools
• Regression_Discontinuity.ipynb: The smoothing and density section is missing

The majority of the models use the statmodels package. While I tried to limit my usage of R code through rpy2 I use it in the synthetic control matching

[kylebutts]

Hi Tom, this is really great! I'm sure there are some python folks who will really appreciate this!

I've moved all of these notebooks into https://github.com/scunning1975/mixtape_learnr/ as it more naturally matches the .Rmd notebooks there. I'm adding the resources to https://mixtape.scunning.com/teaching-resources.html with a big thank you to you. I'm going to close this pull request as I put your work