Add `branches` function

[zachary-foster]

In the group of functions roots, stems, and leaves I think there is a place for everything else, which I was thinking about calling branches.

There is already an is_branch function, so I figure there could be a branches function to make things consistent. It would return info on everything that is not a root, stem, or leaf. The four together would be the whole tree.

[sckott]

sounds good, can you give an example of what a branch would be?

[zachary-foster]

A branch is anything that is not a root, stem, or, leaf. Its the interior of the tree after the first split starting from the roots. I am not sure if its really all that useful, but I like completing the metaphor and it should be easy to implement, since I could program it as any taxa not returned by roots(), stems(), or leaves().

[sckott]

right, does make sense to complete the set