Correct handling of c > 2 coders in agreement metrics
Correct handling of c > 2 coders in agreement metrics

- Fix bug where pi was returning multi-pi (== Siegel and Castellan's (1998) K) with wrong normalisation constant (4 instead of c^2)
- Factor out _pairwise_average whose semantics had been rewritten differently throughout the code
- Provide multi_kappa which averages over Ao and Ae rather than over pairwise kappas

The implementations currently involve individual pairwise computations, which is not as Artstein and Poesio (2007) describe when they instead aggregate agreements for each item, ignoring coder identities. The latter is computed more efficiently. However, the current implementation illustrates more clearly the relationship between metrics for two coders and those for more.


svn/trunk@8818

nltk/nltk,nltk/nltk,nltk/nltk