(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
You should not import nibabel for this. You can simply pass the filename to plot_stat_map.

Also, you should rename the title of the figure. Maybe we should use the term 'Graph-net' here, as it is the term used by many people in neuroimaging.