Improve handling of number attribute in lyric element

[mdgood]

Michael Cuthbert (@mscuthbert) raised the point that the number attribute in the element is confusing, as despite the name it can be non-numeric:

I think it's a bad idea that Lyric's "number" attribute can actually be any character data, not just a number. Can "number" attributes which don't need to be numbers be either (a) restricted to numbers, (b) renamed, or (c) have a strong warning that implementations should not store them as ints?

This issue applies to the number attribute in the lyric element. Any other element attributes with this issue may need to be addressed on a case-by-case basis to determine the best approach between the different alternatives.

See the MusicXML forum discussion at http://forums.makemusic.com/viewtopic.php?f=12&t=2392#p6460.