(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
docstring structures

```
"""Headline

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Parameters
---------------
foo : type
    Description.

Returns
----------
out : type
    Description.
```

Headline fits on one line.