GDP: Rhythms rewrite

[Graham Percival]

I've identified the following areas to work on; please add items or 
discuss them.


- Proportional notation (introduction): delete entirely?  Where
  should we stick links to the real section?
- Time administration: rewrite, or move to somewhere else?

REWRITE
- durations (including redoing the first example)
- multi-measure rests
- grace notes
- manual beams
- general rewrites in Bars


Cheers,
- Graham