trackNumber following 'position' subproperty pattern.
Also https://github.com/danbri/schemaorg/commit/688648974dea6bbdbc5bbe066e696caaa8c44958
Fixes type of trackNumber to be MusicRecording,
declares it a subproperty of 'position', and expects
either Text or Integer, following the model agreed in
discussions around Periodical. See
http://lists.w3.org/Archives/Public/public-vocabs/2014Aug/0133.html