app.nlp.ner: Turning ChainNer2 into an annotator, many cleanups

Since ChainNer2 is the one NER system we have with state-of-the-art performance on conll I think it's desirable to revisit it and make it work in the new framework.

The current situation where its maintenance is "make it compile" is not ideal for something so much effort was put on. This pull request makes it into an annotator and simplifies the code in a way which hopefully preserves correctness.