(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
Lambda is just fine, and is an incredible convenience in such matters. Also, Lambda reminds us of the Lispy origins of python, which most people seem to have forgotten, or completely ignore. Lambda is your f(r)iend. AC would agree.