(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
> I don't mind implementing this. My point was I don't see what added value this
> modification has, appart from being a "'good" practice.

The choice of 42 is arbitrary. To make a check more robust, one might
want to marginalize it.