Add options for different spaCy models / different NER taggers

[elliottash]

Right now, the set of spacy models that can be used is limited. That could be made more flexible. In particular, the package should allow for different NER tagger, eg Stanford NLP’s Stanza library.

-- currently, allows for different spacy models.

[PinchOfData]

Implemented in the branch relatio-v0.3 (method self.process_roles() of the Preprocessor() class)