debug is_bound() against gold standard

[arne-cl]

There seem to be a number of valid candidates which are rejected by the is_bound() filter.

It would be helpful to generate a list of all gold standard anaphora, which are included in self._get_candidates() but get rejected by one of the filters employed by get_filtered_candidates().

• prerequisite: (re)integrate brat/bracketed output generation and coreference chains into discoursegraphs #5