Add inference for new unseen author for `gensim.models.AuthorTopicMod…

…el` (#1766)

* Add function get_new_author_topics() to infer topics distribution for new unseen author.

* Fixes for pep8 compliance. Concrete exception handling.

* small docstring fix

* dont extend self.corpus

* pep8 fixes

* remove chunksize limitation and small fix

* pep8 fix

* try pep8 fix

* convert docstring to numpy-style

* fix PEP8

* fix major bug

* fix pep8

* fix merge

* add tests for get_new_author_topics

* fix stash changes

* fix test pep8

* fix exception catching and rollback

* add test for topic similarity

* fix pep8

* some last cosmetic changes

* author id fix

* add tutorial for authorship prediction

* add ipynb