(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
I would phrase this with something like 'taking in account the fact that the last entry of w is the intercept'.

I am surprised that you are not (optionally) dealing with the mask in this function. I would have thought that, from a code design point of view, this would be a good place to deal with a mask.