MusicXML: Reset measure position on backup elements, print => sys.stderr.write

MusicXML: Reset measure position on backup elements, print => sys.stderr.write

-) Replace all relevant (i.e. non-testing outputs; these are still in there
   in the main() functions for testing) occurrences of print by
   sys.stderr.write
-) If a <backup> element is encountered (e.g. when there are two voices in
   the measure), correctly reset the measure position to the desired
   position after the backup.

Signed-off-by: Reinhold Kainhofer <reinhold@kainhofer.com>