(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
There is an issue with the default alpah_min (1.e-6): in both cases, the decoder selects the highest penalty. I have replayed it with a higher alpha_min: 1.e-4 and obtain clear differences between the two decoders.
The heuristic to set the alpha grid/ choose alpha_min should be revisited ?