ocrd_mets: register our namespace https://ocr-d.de on root node

[bertsky]

We have used mets:note to record the workflow provenance now, in accordance with this proposal.

Alas, currently, we define the namespace prefix ocrd="https://ocr-d.de" ad-hoc on each leeve element. This can cause problems with other XML libraries.

To register the namespace on the METS root element, AFAICS we have to do the following:

1. add it to ocrd_models/mets-empty.xml
2. omit explicit ocrd namespace definition in OcrdAgent.notes
3. ensure it is present on the root element in OcrdMets.add_agent