Adds test for Book.findSegmentFromOffset and cleans up Segment and Section classes

- This removes several unused functions.
- Moved create segment logic from SMILDocument to Segment's constructor
- Book.findSegmentFromOffset is now a wrapper for functions on SMILDocument and Segment
Merge branch 'lyt-3.0' of github.com:Notalib/LYT into lyt-3.0