(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 really do wish it were faster. But it is true that it at the moment it really isn't. In general I only propose einsum to save memory, because it doesn't make copies, not even of stridetricked arrays.