Add causal decomposition

[rafaqz]

See:
https://www.nature.com/articles/s41467-018-05845-7

(no pressure! and not very deep knowledge of these methods on my part, I'm just noting potential algorithms that are missing here as I research the topic)

[kahaaga]

Thanks for opening the issue, @rafaqz! It's nice to have all methods here as separate issues, so someone can work on them if they have time 👍