Fix and perhaps unify calling Java programs

[LourensVeen]

In tasks/_corenlp.py, tasks/_semafor.py and tasks/_stanford_ner.py, Java programs are started and communicated with, in three different ways. The one in _corenlp.py is complicated and has a race condition (self.out may be written to before it is created) and a weird mutual recursion between communicate() and start_corenlp(). That should be fixed, and perhaps it is possible to create a single function or base class that takes care of starting a Java process and communicating with it, and then using it in all these places, in the spirit of Don't Repeat Yourself.