C implementation of Colless index

[jeromekelleher]

Requires #2250

Follow pattern established for #2253

[jeromekelleher]

Python version done in #2266

Would you like to do this one or will I pick it up @jeremyguez? It's slightly different to #2253 because we can use the built-in postorder rather than using a stack.

[jeremyguez]

I can have a try at it, it would be a good experience. I'll look at #2253 and will tell you if I get stuck somewhere.

[jeromekelleher]

Any update here @jeremyguez? We're hoping to release Python 0.5.0 in the next few days, so if you're not going to get to it soon I can do the C updates.

[jeremyguez]

Sorry @jeromekelleher, didn't know a release was coming soon.
I started to dive into the C code, but being at end of my PhD with the writing of the thesis/papers, I don't have much time at the moment. 😞
So yes, you can go ahead and do the C updates. I'll look at it when I have more time, to understand how you made it work. 😃

[jeromekelleher]

No worries at all @jeremyguez, thanks for being so helpful with the rest of the implementation!