(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
You need to specify the expected shape of X. I cannot tell from looking at the function's signature and docstring if it is expected to be 4D (volumetric) or 2D.