(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 don't think that a function should be called "my_alpha_grid". The name isn't quite as explicit as we would like it to be. We need to find a good term to replace the "my_". What is special about this alpha_grid?