Create econometrics tutorial notebook

[Varnasr]

A guided Jupyter notebook walking through all 5 econometrics methods using the sample_program_data.csv would make the module much more accessible.

Structure:

1. Load and explore sample data
2. DiD estimation with parallel trends check
3. PSM with balance diagnostics
4. RDD with bandwidth selection
5. IV/2SLS with first-stage diagnostics
6. Sensitivity analysis