Linear (regression) models for Python. Extends statsmodels with Panel
regression, instrumental variable estimators, system estimators and
models for estimating asset prices:
* Panel models:
  - Fixed effects (maximum two-way)
  - First difference regression
  - Between estimator for panel data
  - Pooled regression for panel data
  - Fama-MacBeth estimation of panel models
* High-dimensional Regresssion:
  - Absorbing Least Squares
* Instrumental Variable estimators
  - Two-stage Least Squares
  - Limited Information Maximum Likelihood
  - k-class Estimators
  - Generalized Method of Moments, also with continuously updating
* Factor Asset Pricing Models:
  - 2- and 3-step estimation
  - Time-series estimation
  - GMM estimation
* System Regression:
  - Seemingly Unrelated Regression (SUR/SURE)
  - Three-Stage Least Squares (3SLS)
  - Generalized Method of Moments (GMM) System Estimation