(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'd like this function to take an explicit verbose parameter, to replace the one that it is currently hiding in the solver_params dictionary. The way it should be handled is that it should be passed raw to the EarlyStoppingCallback, but to the solver, it should be passed as follows:

<pre>
verbose=max(verbose -1, 0)
</pre>

that patterns enables to control the amount of verbosity in a pipeline.