(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
> Are we supposed to use double/single backticks etc for the docs or not? I have
> seen them in several places, but not consistently.

Only if there is an underscore at the end of the variable name, as
without the backticks, it is interpreted as a link.