(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 don't like this. One aspect is that "the volume of the MNI 152 brain" is very accurately defined. Second,  the user may have a hard time guessing what is actually done (some datasets are not full-brain, henc ethe user will have to figure out what fraction of MNI space their mask represents). I think that the setting is clearer is the percentile is wrt the provided mask.