(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
Because parameters can be changed later by user, we usually do these checks at fit time.

If you want to do them both at construction time and at fit time, you can define a _check_params method that does these checks, and run it at the end of the __init__ and at the beginning of the fit.