(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
@dohmatob any arguments as to why? What are the time scales of the necessary operations these days?
@lesteve a `w` instance would contain a brain image, potentially masked, but I am not sure at this point. So its shape can go from around `(10, 40, 40)` to `(100, 100, 100)`. As for `max_iter`, I am guess a couple of hundreds, but @dohmatob has more experience with that.