Fix some bugs (details in comment)

1. In Basic/WindowClassificationPerformanceEvaluator:
init Kappa array with inst.numClasses() instead of classVotes.length since classVotes.length is varied for different instances and can cause the initialized Kappa array to be shorter than required.

2. In ArffFileStream:
reset lastReadInstance to null in prepareForUseImpl() to make sure next time nextInstance() is called, lastReadInstance will be reset to the first instance in the stream.