(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
> scipy's expit is buggy (not well coded) and can encounter overflows.

1) Does it matter for what we do? Ie: will we encounter these situation.
2) Is your code more robust to overflows?