(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
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> But precisely, ANOVA (one-way) seems to be what's been done under the hood in the scikit (i just rushed into the code at the moment).
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there are different ways. It depends what nilearn supports.