(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
@GaelVaroquaux do we need to clip in any of this code? Seems to me like some positivity constraint stuff that was used in other contexts, but doesn't seem to be called on here. (Certain future use or out?)