Add TreeSequence.coiterate(other, **kwargs) method

[jeromekelleher]

As discussed in #815, this is a useful operation and should be part of the public API.

The implementation is here, which I think is probably fine. We can leave the door open to coiterating along more than two tree sequences via *args, but we don't need to worry about doing it for now.

We just need a handful of tests to make sure that (a) this does what we expect on a simple example; (b) the trees are returned in the correct order (i.e., tree1.tree_sequence is ts1) and so on; (c) we test some cases like one ts has one tree and the other has 3, etc; (d) test error conditions are handled correctly.

I think we should get this in for 0.3.3

@benjeffery @hyanwong, would one of you mind picking this up please?

[hyanwong]

I can pick this one up if you like. Something to do while waiting for rescomp to come back up. Should be easy I think.

[jeromekelleher]

Great, thanks @hyanwong.

[hyanwong]

Shall I put it somewhere like tskit/util.py or have it as a method on a ts in trees.py?

[jeromekelleher]

I think it's a method of TreeSequence: TreeSequence.coiterate(self, other, **kwargs)

[hyanwong]

Which reminds me, trees.py is getting pretty long now. I wonder if we should split it into tree.py and tree_sequence.py? If you think this is a good idea @jeromekelleher I'll open it as an issue.

[jeromekelleher]

Feel free to open an issue to gather opinions.