MIGRATE: move stats code to statsmodels / deprecate in pandas
related #5884
- [ ] OLS https://github.com/statsmodels/statsmodels/blob/master/statsmodels/regression/linear_model.py
- [ ] plm (regression for panels)
- [ ] fama_macbeth
- [ ] VAR https://github.com/statsmodels/statsmodels/tree/master/statsmodels/tsa/vector_ar
