(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
We need to smooth the data that we are passing to the SelectPercentile here, using nilearn.image.image._smooth_array with fwm='fast'. PR #258 will enable you to pass affine=None there.