Topic/dictionary indexing tweaks

I noticed some dead code in one of the rename functions, and figured I would send a PR rather than make a comment (and add changes for my other comments - which you can choose to ignore).
I'm actually slightly worried about this, I actually thought the dictionary had both the new _and_ old feature ids, but everything works fine without it.

My concern is mainly around the skew calculation, which is probably wrong at the moment.