(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
No need to define a variable if you are using it only once and not modifying it. You should simply inline '.5' in the code. For someone who reads the code, it is easier, as he does not have to scroll between variable definitions and the lines where the variables are used.