Split LambdaPoutine into ScalePoutine + IndepPoutine

[fritzo]

This is a pure refactoring PR.

Why?

This will make it easy to scale different parts of a dataset, e.g. in stratified sampling for fraud detection.

[martinjankowiak]

lgtm if tests pass. while we're at it we should probably get rid of "map_data_stack" everywhere

[fritzo]

@martinjankowiak Mind if I do the map_data_stack cleanup in a separate PR? I'd like to rename it to cond_indep_stack and reverse it (to be more pythonic).

[martinjankowiak]

@fritzo i suppose that's ok... seems awfully indulgent with our limited travis cores though ; )

[fritzo]

Ready to merge.

[fritzo]

Thanks, Martin!