(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
That test is too stringent. In practice, it pretty much never fires and models are optimized to convergence during the parameter selection period, which takes for ever. I would change it to '>= -1e-4'. This will lead to a great change in run-time for no change in results.