Average rank computation

[Didayolo]

It seems that we can do the following columns computation: sum, avg, min and max.

See https://github.com/codalab/codabench/wiki/Leaderboard-Functionality and the screenshot below.

However, we can't compute average rank? It is a very important computation. Also, as Codabench is supposed to be v18 compatible, I though average rank was working already. We should implement it.

https://github.com/codalab/codabench/blob/develop/src/apps/leaderboards/models.py

In the long term, having organizers columns computation is what we want (#406), or at least being able to chose from the ranking functions of Ranky.

[Didayolo]

I close this issue to keep track of it only in #406