DocExtract: several improvements

* Accepts Publications as reference section.

* Refactors book handling and search.
  (closes #959)

* Daemon extracts all added pdfs.

* Correctly handles PDF/A files.

* Adds LIGO and LHCP report numbers to the kb.

* Adds the filename of the extracted files in the 999C6f field.

* Converts arxiv urls to report numbers

* Adds support for hdl identifiers.

* Sets a priority on which title form is the most important.
  References will be given priority over anything else while
  Reference will rank last with Bibliography in between.
  That means that if "Bibliography" is found in the text, we assume
  it is the reference section and we don't look for a section titles
  "Reference".

* Improves error message when pdftotext is missing.

Signed-off-by: Alessio Deiana <alessio.deiana@cern.ch>
Tested-by: Samuele Kaplun <samuele.kaplun@cern.ch>