(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
Please change **params to solver_params, and use a dict rather than a '**' pattern. We have a policy against '**', as they make forward evolution hard. Here you can just use a dictionary, that you pass to your solver using '**'.