(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
How about we never deal with the case of no mask (ie mask becomes a mandatory argument), and write the code above as:

<pre>
w = w[mask]
</pre>

This would also remove the need for raveling.