(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
> > +def test_grad_div_adjoint_arbitrary_ndim(size=5, max_ndim=5, random_state=42):

> private function?

Should be in tests.