(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
OK. When we're done with this mask wizardry, we'll barely need any spatial
prior whatsoever: we could simply fit a vulgare ridge on the then highly
structured support :)