Rethinking spanners and number levels

[rpatters1]

After digesting the solution for #320, it occurred to me that I should at least float an idea for another way to do this. Since the goal of mx::api seems to be to create semantic abstractions for musicxml data, a cleaner way to handle spanners might be to model them the way mnx, MuseScore, and (to a great extent) Finale do. (These happen to be systems I am familiar with.)

That is, the client would store everything about the spanner in the spanner data for the starting note. This would include the measure and tick position where it ends, and/or the end note when necessary for some spanners. This approach has some obvious challenges.

• how to identify the end note
• difficult to roundtrip with identical numberLevel values

It is such a foreign concept to musicxml that I did not suggest it in my original issue. But I've been biting my tongue ever since. It looks like the current solution will work, but I finally decided to throw the idea out to be considered.

[webern]

That does sound like a better model, overall. I'll give it shot at some point.