fix validation errors with MEI v3.0.0

validating the data folder against https://music-encoding.org/schema/3.0.0/mei-all.rng reports 2875 errors. Skimming through the report there are various types of issues (e.g. HTML elements inside MEI, missing fragment identifiers) and some errors might eventually shadow even more errors.

The initial report as HTML: validation-results.html