Create a complete example reference list in XMP

[bencomp]

Create an example sidecar file that demonstrates what the XMP contents should look like, before trying to create such files on the fly.

Assert that the file is legal XMP.

Example should contain

• multiple authors, title, publication, publisher, ...
• persistent URI for the document
• licence information
• reference to journal article
• reference to conference or workshop paper
• reference to book
• reference to versioned webpage
• reference to dataset
• info on citations among these references