Add preorder and postorder functions

[jeromekelleher]

We can't rely on having the preorder and postorder methods from tskit anymore, so we'll need to have a way to compute them (returning arrays). Preorder is easily done with a stack. We can do postorder using recursion for now for simplicity.

We should also consider defining these as dataset variables, so we reuse them if we want to.

[benjeffery]

We should also consider defining these as dataset variables, so we reuse them if we want to.

Might be time to have a specification document somewhere for the dataset? Given this and the MCRA array?

[Billyzhang1229]

We should also consider defining these as dataset variables, so we reuse them if we want to.

Might be time to have a specification document somewhere for the dataset? Given this and the MCRA array?

Yep, I think we should write down the variable definitions may be in a markdown file?

[jeromekelleher]

SGTM - we can formalise more later.