Fix: Xml documents with multiple score parts were not read anymore

[gatzsche]

Hello :-)

In the last update you introduced an exception when an xml document contains more then one score-part. This is problematic for my use case working with documents having multiple score parts.

I removed that restriction.

Please let me know, if this conflicts with other requirements.

Best, Gabriel

[thaihuynhxyz]

Thanks for fixing it.