(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
Averaging is done after joining all the fold workers . This re-fit here is simply to solve the best model to high precision to reduce the noise / impression in the maps induced by early stopping, etc. Plus this only adds worst-case complexity O(n_folds * hardest of high precision problem), which is reasonable.