musicxml2ly does not recognize clef elements containing number attributes

[Patrick Schmidt]

Hey all,

here is a tiny example (see file attached) to demonstrate the issue: the 
musicxml2ly-conversion results in a common treble clef instead of a bass clef. 
Without the number attribute everything works as expected. MusicXML allows 
number attributes in clef elements, so it's valid XML-code.

hth
patrick

bass-clef.xml
Description: application/xml