(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'd prefer 'classif' to be named 'is_classif'. We don't have a docstring here, which I think is fine because it is a very internal object, but we need to be careful that people can guess the function of the argument as easily as possible.