(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
Maybe this test can be revived using `assert_array_almost_equal` using a lax `decimal=` kwarg. But that depends on how bad the differences are when using early stopping. If the smooth lasso with `l1_ratio=1.` and the TV-L1