Tfidf whitespace-related fixes

These commits undo the accidental damage to the whitespace in the BM25Weight code, fix a build error (due to trailing whitespace after the ```\``` in the new line in weight/Makefile.mk, and remove all trailing whitespace from your changes.

It's generally best to avoid having trailing whitespace - sometimes it is significant (e.g. after a ```\``` in a Makefile or shell script) and because it is invisible, it can be hard to spot what the problem is.

If you have coloured diffs on in git, then "git diff" will highlight such whitespace by default.  It's a good idea to look over the changes you've staged to commit with ```git diff --cached``` before committing them, which will show you such problems, and also make it obvious if you've included any changes you didn't mean to.