(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
Could you define a _test_score_classif and _test_score_regression outside of the function. Defining functions inside other functions isn't great as it doesn't encourage their test / reuse.