Should Input/Output nodes be first/second children instead of first/last?

[cqc-alec]

This would make it slightly easier to add new non-IO children.

[ss2165]

@acl-cqc Do you have any objections? I think this is a good idea

[aborgna-q]

👍

[acl-cqc]

Not really. I think we could do with a utility method like your boundary that returns a pair/array<2> of Nodes given the DFG container, but given that, fine

[ss2165]

Issue #146
PR #148