(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
>   • Yes it is fully envisageable to encounter such situations in real life (at
>     least I don't see how we'll bulletproof from it)
>   • At least for the moment, I've not had overflows with my code. Thus I'd
>     consider the switch fragile (plus the expected gain in speed is
>     questionable). There is probably room for optimization.

Your code doesn't do anything to avoid overflows. Thus unless there is
good evidence that it does better than scipy, I don't see a reason to use
it. What makes you think that it does better than scipy?