(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
Could you please:

a. Not keep the return value: it is not used here, so let's not clutter the script

b. Lock the cut_coords to z=-16, y=-38, x=35 so that the images are comparable with other images in the docs.