Handling mediapackage XML needs rewriting

[JamesUoM]

GC currently rewrites all mediapackage XML, even that which it harvests from the Opencast scheduler. The implementation does not use a DOM for the full process and this leads to problems with the XML being changed or additions to the default catalog not being supported. Though ingest of scheduled recordings merge the MP with what is in Opencast re-ingesting uses only what GC has written, which means recovering from failed initial workflows is being compounded by further issues:

1. The episode XML is converted to a dictionary, which can not handle multiple elements of the same type eg more than one dcterm:creator elements, lost metadata https://github.com/teltek/Galicaster/issues/460

2. The start attribute of the MP (manifest.xml) is rewritten from the episodes created field and no longer matches the intended scheduled start. This is often a trivial 1s earlier but can cause knock on effect when this start timestamp is used.

[Alfro]

I'll take a look at this.