(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
Let's use 'decoder' as a variable name here, rather than 'slcv'.

Also, let's split the instantiation of the object and the call to fit in two different lines: it will be easier for the reader.