Allow multi-class sequence tagging

[johann-petrak]

Ideally by allowing a list of class annotation types, or/and by allowing to use the targetFeature to distinguish between several classes.

[johann-petrak]

This is now partially implemented. Leaving open until we have implemented at least one or two alternatives to BIO and until we have implemented some tests for encoding odd overlaps and decoding all possible sequences (there may be the necessity to parametrize how to handle certain cases).