(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
in the three above blocks, the `def` statements would be better inside the `if` blocks (they don't need to check `loss` every time that way). Some editors will claim redefinition of function names, but that is not the case.