(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 believe that we should try to get rid of 'common': it is somewhat of a generic file which has a non descriptive name. We could start by moving 'check_lipschitz_continuous' in the 'fista' file: is this function used anywhere else?