(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
> > +        self.__class__.__name__.endswith("CV")

> ???

Just don't catter for non CV stuff. In the nilearn codebase, the way to
do non-CV stuff will be to give a single value of alpha and l1_ratio to
the objects.