New OncoPubs exporter

[mbaudis]

The changed document structure in the publications collection requires a new exporter script:

• implement in Python
• export as single YAML documents
• options
  • provide input filter (PMID list, for a start)
  • use a template for output formatting (e.g. a schema file or ordered list of primary parameters) to provide consistent order in the output documents

A first (urgent) application is the deposition of the publications from the "cancer geographies" manuscript's analysis into the supplements repository.