(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
There is indeed a name collision between the image spatial gradient and the derivative of a function.

With Gaspar we had decided to call gradients in the sens of derivatives 'derivative'. This is not standard, but removes the confusion. In addition, here, we could use the term 'img_gradient'.