Write LASSO chapter

[hschafer]

• [x] Recap overfitting + regularization
• [x] Feature selection overview (benefits, interpretability, sparisty)
• [x] All subsets (an efficiency)
• [x] Greedy algorithms
  • [x] Forward stepwise
  • [x] Alude to other forms
• [ ] Regularization with Ridge for feature selection
• [ ] LASSO
• [ ] Demo
• [ ] Sparsity (why?)
• [ ] Choosing lambda (same)
• [ ] Practicalities (debias lasso/feature selection)