(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
Oops! Indeed, we're in a loop, and so *.copy() is not "just fine", as the overhead will become apparent for large "max_iter". Good catch.