(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
Again, TV (a penalty) is mixed up by juxtaposition with S-Lasso (more than a penalty) -> Adds to confusion for new people in the field