(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 don't understand the logic at play here.

1. If no `err_msg` argument is provided, it seems to me that in err_msg the `x` and `y` will be set to the first x and y in the loop rather to the ones that don't satisfy the constraint, potentially yielding to a misleading error message. In other words, I would move this statement in the `if a > b:` block below.
2. I don't really get the point of being able to provide an err_msg as an argument to be honest.