have a special BibTeX cache_key to avoid reusing the current processor for the bib conversions; Infinite loops lurk in the dark otherwise!
 allow for biblography objects passed from the combined core+post API
 ensure we empty the bibliographies for the BibTeX conversion, to ensure no infinite loops occur
Merge branch 'inline_bib_fixes' of https://github.com/dginev/LaTeXML into dginev-inline_bib_fixes