[MRG] Fixes against #2698

[piskvorky]

Fixing some low-hanging fruit in @gojomo 's epic PR piskvorky#2698.

• Resolve conflicts and merge develop.
  • Removed the recently added max_final_vocab; will need to look deeper what's going on there, following here.
• Many code style fixes (hanging indents, .format etc).
• Fix segment_wiki.py
  • This file had Windows line endings (??), my git refused to commit the file => converted to unix here.
• Resolve comments in the original KeyedVectors & *2Vec API streamlining, consistency piskvorky/gensim#2698 that were already fixed by this PR.
  • Only to-be-resolved comments left.

I have a better sense of the changes and direction in piskvorky#2698 now. Several comments remain unresolved, but I find that PR impossible to work with on Github. It's so large I can't even track notifications – I have to manually expand all files and look for what changes, new comments… 😱. How do you guys do it?

So I'd propose:

1. Fix critical issues that block merging here. I think there's just one but @mpenkov hasn't reviewed fully yet.
2. Make sure tests pass (@mpenkov please see KeyedVectors & *2Vec API streamlining, consistency piskvorky/gensim#2698 (comment)).
3. Merge this PR.
4. Merge KeyedVectors & *2Vec API streamlining, consistency piskvorky/gensim#2698.
5. Copy over unresolved questions, comments, TODO lists – somewhere where they're easier to track.
   • I personally use FIXME to denote sections that must be resolved before a release (like critical bugs, stubs) & grep for FIXME before a release, to verify none are left. And use TODO for things that can be released later, incrementally, more "wish list".
6. Continue addressing TODOs in new (smaller, focused) PRs: renaming, refactoring, docs…
7. Release 4.0