Humdrum export: At start of export, fix up various duration problems (seen e.g. after
import of a Photoscore-generated MusicXML file) that would prevent Humdrum export
from succeeding.
Humdrum/MEI export: There are a lot of custom attributes that we add to various objects
in the score to keep track of export status. Remove them at end of export.
MEI export: Put xml:id on (almost) everything in the exported MEI file.
MEI export: Clean up ChordSymbol support.
MEI export: There is no `heavy-light` barline in MEI. Rather than fail when we try to
export one, just export a normal barline.
MEI export: If voice.id is a string, put it in `layerlabel`, not `layern` (which must be a
number).
Shared: SplitComplexRestDurations can now handle rests that have a different visual and
gestural duration (in music21 terms, these are rests with an unlinked duration).
Shared: M21Utilities has a lot more stuff in it now. Some is moved from other places in
converter21, so it can be shared with other parts of converter21 (or even with clients),
and some is new (like the new fixupBadDurations API, used by the Humdrum exporter; see
above).
All: lots of little bugfixes.