(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
Also, I would prefer that you implement this as a function, or maybe even in-lined in the right place, as in the SparseVariation codebase. It helps reducing the complexity of the codebase.