(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
Which `ImportError` are you catching? nilearn depends on sklearn, so it shouldn't be necessary. If I am mistaken, then at least change `pass` to `raise SkipTest` (or something to that effect), because `except: pass` is bad practice