NR 2.8 Ancient music ready for review

[Eyolf Østrem]

The section on Ancient music is now ready for comments. It has been
completely rewritten and reorganized, so please have a look at it and
comment, correct, or criticize.

Here's a "changelog":

- The main change is that the general organization has been revised into
  two main sections: Gregorian chant notation and Mensural music. There was
  fairly little overlap between the alternative signs, which were mostly
  ancient, and the additional, which mostly belonged to the Gregorian
  section. 
- The two ligature tables in the Gregorian section have been merged into
  one. 
- There are still a few TODOs left, as well as missing cross references
  here and there, the odd example of bad formatting, etc. This is
  particularly true about the last section, which has not yet been written.
  I focused on the main text for this first round; there will be another
  round for this kind of formalities later on.

Eyolf