Code style fixes to the TFIDF module

[piskvorky]

That module is ancient, didn't adhere to the current gensim code style.

[piskvorky]

@tmylk how far did you get, making the gensim code base style consistent?

@menshikh-iv as Lev probably explained to you, we want to keep the code style consistent. We don't want to merge any new PRs that break this style. Plus we want to fix any existing code style lapses (such as in this PR).

[menshikh-iv]

@piskvorky Where can I read about code style in gensim project? (in developer page I see information about spaces only) ?

[tmylk]

@piskvorky There is a hook at every commit that runs the flake8 check on the touched file. That way the code style will improve over time. The hanging indent check is being reviewed by pycodestyle project PyCQA/pycodestyle#632 . It will become a part of gensim's automated checks when it is merged.

BTW This is not a strictly style fix and I would rather omit it. In general having 2 attributes id2word and dictionary with identical function is confusing, but should stay to support backwards compatibility.

[piskvorky]

I don't think there is a dictionary attribute, so there no duplicity. Or what do you mean?

[piskvorky]

@menshikh-iv it's pretty much PEP8 and PEP257, except we ignore the strict "80 char" line limit (80 is just a recommendation; if the break would split a line unnaturally, making it hard to read, we use any other limit... 100, 120, whatever).

Also, we use hanging indent (not vertical indent). Both are OK according to PEP8, but we just use hanging indent for consistency and because it's easier to maintain.

I think that's it (@tmylk?)