(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
unless C ordering guaranteed, there is a risk of a copy here (but it should mostly be C-ordered). Can we still have that the reshaping logic happens in the `total_energy` function? It feels more consistent.

Also somehow my previous comment on the legibility of this loop has disappeared. Since I have parsed it now, I cannot comment anymore on how simple or difficult this is, but my first reading would have been better guided by a short explanatory text before the loop.