Non-validating files

[rettinghaus]

It is not possible to validate the files, because the DTDs are pointing to http://www.musicxml.org/dtds/partwise.dtd, i.e. to nowhere.

For the time being we should replace them with https://raw.githubusercontent.com/w3c/musicxml/v4.0/schema/partwise.dtd (or whatever version seems appropriate).

relates to w3c/musicxml#480