Define how to encode instrument transposition

[joeberkovitz]

This issue notes the need to represent the transposition of instruments including both key changes and octave shifts.

This is related to the more specialized concerns in #4 and #34 but concerns the general nature of the transposition.

[clnoel]

I am commenting here to draw attention to the fact that there is an issue already to deal with these specifics, since part of the proposals in the general issue #138 include the addition of a <transposition> open-ended direction, which can handle several of the cases here. We should probably discuss here how to use that in conjunction with visible octava, key-changes, or clef-changes.

[adrianholovaty]

See pull request #158 for some work on octave shifts.

[adrianholovaty]

(Just renamed this issue to make the intent clearer.)

[adrianholovaty]

Marking this closed in favor of #287.