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.