(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
Why is there an 'l1_ratio' variable here that is not used?

Why is 'tau' called 'tau', and not 'alpha' as we do in standard?