(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
> OK. When we're done with this mask wizardry, we'll barely need any spatial
> prior whatsoever: we could simply fit a vulgare ridge on the then highly
> structured support :)

The important aspect is to select a super-set of the voxels that matter.