(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
Maybe I'm wrong, but that basically excludes all ROI based analyses from benefitting from sensible out-of-the-box settings, right? (I am not mentioning single subject analyses as an argument, but one may want to accommodate those as well in the future.) As @bthirion says, a *volume* percentile of a specified mask seems as sensible to me as selecting a percentile can get in the context of this algorithm.