(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
In #241, we agreed that calling _smooth_array with fwhm='fast' was the way forward because it has a little bit of additional logic, like getting rid of non-finite values in the array. It may apply to this case too.