(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
meh - if I may: This function name sounds like it kind of stops the `prox_l1` from doing something somewhere in the middle. `prox_l1_skip_intercept` ?