Add a way to encode visual style of half note multi-note tremolos

[adrianholovaty]

Just creating an issue to keep track of this outstanding to-do from #454: we should add a way of encoding the visual style of a multi-note tremolo.

The primary case for this is tremolos in which the displayed noteheads are half notes. Some publishers prefer these to be beamed with a single beam (while other beams are disconnected), others use all beams connected, others use no beams connected.

We could provide a way to encode this decision, both on a per-multi-tremolo basis and at some global level. I didn't implement it as part of the #454 changes because I wanted to wait until we had a way to encode global styles.

[rpatters1]

A related issue is whether individualDuration should override the duration value in outer. Or if the two values don't agree, is it an error? This seems to run against the design goal that MNX only has one way to encode something. Or perhaps there is a reason why they might be different? If so, that should be documented explicitly.

I am also wondering why the name outer (other than that it matches the name in tuplets). The name doesn't seem particularly pertinent to tremolos. What about something like fullDuration, or just duration (especially if it were determined that individualDuration is redundant.)

Finally, is there any actually use case for multiple other than 2? Even Kurt Stone does not show one, and if it existed I would expect to find it there. If there is none, I don't think the design should allow it.

EDIT: On the last point, never mind. Elaine Gould shows examples of 3- and even 4-note tremolos on p. 229.

[rpatters1]

Another issue: what happens if event durations do not match the value in individualDuration or outer.duration? It seems like an error. The design allows for a lot of discrepancies.