Add support for relative threshold

[billbrod]

For Synthesis (and its subclasses), we have two stopping criteria: either you reach max_iter or your (absolute) loss decreases by less than loss_thresh over the past loss_change_iter iterations. But this is an absolute number, which is going to differ wildly depending on the magnitude of your loss.

Would like to add support for a relative threshold, rel_loss_thresh, which checks whether (absolute) loss has decreased by rel_loss_thresh * loss_prev over the past loss_change_iter iterations. On each iteration check if loss < loss - rel_loss_thresh * loss_prev and, if so, update loss_prev = loss. Keep going until there have been loss_change_iter iterations without that, and then break.

This would go into Synthesis._check_for_stabilization, and would need to do a similar check with coarse_to_fine