<harmonic> misses a number attribute

[lemzwerg]

Gould shows examples of both artificial and natural harmonics that occur more than once in a chord:

I thus suggest that <harmonic> gets a number attribute similar to other elements like <slur> so that base, touching, and sounding pitch can be grouped together. Especially the situation in the right chord of the second image can't be correctly resolved otherwise in an automated way.

[mscuthbert]

The W3C co-chairs and I have discussed the issue and are in agreement that this is needed for these cases. It will remain optional (and an empty entry will default to 1). Will be adding to the 4.1 milestone.