axml - embedded XML document for Broadcast Wave Format

https://tech.ebu.ch/docs/tech/tech3285s5.pdf

> The `axml` chunk may contain any data compliant with the XML 1.0 format or later, a widespread 
> format for data exchange [4]. Note that the <axml> chunk may contain XML fragments from more than one Schema.
> The `axml`  chunk may occur in any order with the other BWF chunks within the same file.
