Revise 14: Integrating MEI with other Standards and Formats

[kepper]

This issue can be used to discuss chapter 14 of the guidelines.

[craigsapp]

Changes from MEI 3.0 to MEI 4.0 and other edits to do for chapter 14

• @dur.gesvalues using PPQ tick durations should now be changed to @dur.ppq for MIDI in particular (but also applies to MusicXML <duration>, so probably add a section for MusicXML to discuss that).
• Maybe mention @dur.real and relation to MIDI.
• @midi.instrnum has changed significantly in MEI 4: Now it is 0-indexed, and can allow 1-index instrument enumerations by prefixing a number with in (meaning "instrument"), such as in1 for piano, which is equivalent to 0 as a plain integer. It would probably be useful to add a list of all instruments and their representations as unitless integers, "in"-based integers and a strings (which are stored in @midi.instrname)
• MIDI channels have also changed (I will review the current definition to see if it is what I remember).
• Probably add a section on Humdrum for discussion of @dur.recip