add weak span based f1 metric

[Bhuvanesh-Verma]

This PR adds SpanBasedF1WeakMeasure metric. The SpanBasedF1WeakMeasure implements span-based precision, recall and F1 measure for different tagging (encoding) schemes. A span can either have exact match (strict) or it can be partially overlapped (weak) with predicted span. This metric allows both strict and weak version with the help of a boolean parameter weak. The concept of weak and strict span overlap is described in Lauscher et al. (2018).

This metric can handle multiple encoding schemes (available here) using a parameter called label_encoding. Finally, this metric can retrieve not only the F1 measure but precision and recall as well. Using a parameter called return_metric, we can specify if we want the metric for each tag or averaged over all tags.