(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
as much as I tend to like writing things like this myself, could we have

```
end = grad.shape[0] - 1
if l1_ratio  == 0.:
    end += 1
```