(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
The naming convention is to end names of variables that are niimgs with '_img', not '_niimg'. So that would be 'coeg_img'. Please also correct all the other variable names of this script.