(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
Sorry, it's an upper bound. OK. The difficulty was elsewhere and is not solved by calculating this exactly. But still, maybe it can be replaced by using the operator norm (\infty, 2)?