(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
there is little logic in the namings here: there is a "-" between smooth and lasso, but not tv and l1. I think that I would prefer the parameter to TV-l1 to be "tv-l1"