DAISY 2.02 Validator

This is a port of the "Daisy 2.02 DTB Light Validator" included in Pipeline 1. This implementation does not yet support validation of the MP3 files themself.

I've developed this on the same branch as the EPUB3 to DAISY 2.02 script, which is why the commit log contains some of those commits. However, this PR only contains the DAISY 2.02 Validator script and can be merged independently. The EPUB3 to DAISY 2.02 script will be in a separate PR.

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Nitpick:  Not sure if this was copy-pasted from Pipeline 1, but push-style `template`s are more readable and maintainable than pull-style `for-each` IMHO. OK to use `<xsl:template match="h1|h2|h3|h4|h5|h6">` instead ?