Add entity coreference

[ahalterman]

@philip-schrodt is working on adding support in Petrarch2 for the entity coreference information that CoreNLP outputs. This should increase yield for later sentences in a story. Feel free to add details below, Phil!

[philip-schrodt]

In the course of working through the --nullactors code, I noticed that PETR-2 already has an assortment of rules -- pretty much the same as were used in TABARI, albeit probably from a different source -- for resolving pronouns within a sentence. That's not the same as SCNLP's coreferences across sentences but it's a start. Quite a long while ago JWAC (pre-sequester, when there was a JWAC...) did an evaluation of the TABARI rules against some more sophisticated syntactic systems (IBM was one) and found they worked just as well: problem, of course, is that the issue is fundamentally semantic and syntax will only get you so far. I digress.