Add RankCorr directory

Initially I wanted to add this via git submodule but the git repo contains pycache files which change every time RankCorr is run! I have therefore simply downloaded the repo, removed the offending files, and commited the rest. Another option would be to git submodule a fork.

Ulimately, it seems unlikely that RankCorr will be updated soon so the download and commit strategy is probably sufficent.
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