(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
> Smoothing is already done in the NifitMasker.

We need to disable it: we should never smooth when there is a spatial
penalty. Thus, for SpaceNet, we don't smooth, aside in the feature
selection.