(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.