Update metrics. R

[theiling]

Corrected rare case, where metrics fails and gives "NaN".. Happens in case of perfect match and both expected.mat and confusion.mat only have one nonzero entry. (Example where the metric fails: rater.a = c(2, 2, 2, 2, 2, 2) rater.b = c(2, 2, 2, 2, 2, 2)) Help by Stephen McInerney in Kaggle forum