implement WeakSpanBasedF1Measure #22
Description
Add following modules
- sequence_tagging.ill_formed_tag_sequence (from here) : Encoding #28
- sequence_tagging.encoding (from here) : Encoding #28
- sequence_tagging.annotations (from here) : Convert annotation #29
- metrics.weak_span_based_f1 (from here) add weak span based f1 metric #32