(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
@eickenberg  @banilo : Gentlemen, could you be less superficial and impertinent in your comments ? This is a headliner saying in the most succint words, what the (private low-level) function is doing. It's by no means an *ouroboros*. It's a pity, such an improbably heavy bail of poor reviews drown a few interesting comments which actually have a point to make. I know it's a delicate art to make good reviews, but if you can make an effort,  then we can see some better reviews here. Looking forward to this.