(WIP) Sparse models: S-LASSO and TV-l1

- Supports TV-l1 and S-LASSO priors
- Supports logistic and squared losses
- Has cross validation
- Can automatically select alpha by CV (+ automatic computation of useful alpha ranges for the CV)
- Warning: User must supply l1_ratio
The code snippet you've highlighted is essentially the code for the scoring logic (at least the regression case). Are you saying you want me to embed scoring logic into early stopping logic ?