(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
Same thing here: you cannot modify sys.path.

This might mean that the functions in simulate_smooth_lasso_data should be moved in the nilearn package, where they are importable.