Add gensim Author-Topic-Model support

[jonaschn]

This PR adds the ability to use the Author-Topic Model in the same way, the LDA model is used.
It does not visualize the authors but only (as expected by pyLDAvis) the topic-term distributions.
See #112 and #84 for more details.

[bmabey]

Sorry, I am just seeing this. It looks like there are conflicts due to some recent PRs that have been merged. Would you still like to see this PR merged? If so would you mind resolving the conflicts? Thanks!

[jonaschn]

@bmabey Thanks for asking. Sure, I would like to see this PR merged. I resolved the conflicts.
However, I am not sure if the value of the passed (and required) parameter rhot is appropriate.
@tmthyjames proposed this value.
I asked the devs of gensim what is appropriate: piskvorky/gensim#1766

[msusol]

I have a new vs 3.3.0 ready to push to git, please hold until after this version is official.

[msusol]

Notice: I had to rename gensim.py to gensim_models.py so you will need to reapply your fix and retest.

[msusol]

Please resolve the conflicts before I can complete the PR.

[jonaschn]

The proposed solution in #112 works only if there is only one author per document (the second test case).
I still need to figure out how to support other scenarios as well.