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>
1: D400 First line should end with '.', not 'k'
32:29: E126 continuation line over-indented for hanging indent
34:28: E126 continuation line over-indented for hanging indent
43:36: E126 continuation line over-indented for hanging indent
49:37: E126 continuation line over-indented for hanging indent
54:31: E126 continuation line over-indented for hanging indent
58: D101 Docstring missing
62: D290 White spaces around docstring should be trimmed.
62: D400 First line should end with '.', not 's'
62: D205 Blank line missing between one-line summary and description
77: D290 White spaces around docstring should be trimmed.
77: D400 First line should end with '.', not 'k'
125: D103 Docstring missing
181: D103 Docstring missing
195: D103 Docstring missing
216:1: E303 too many blank lines (3)
216: D103 Docstring missing
230: D401 First line should be imperative: 'Construct', not 'Constructs'
235:9: E128 continuation line under-indented for visual indent
236:9: E128 continuation line under-indented for visual indent
237:9: E128 continuation line under-indented for visual indent
238:9: E128 continuation line under-indented for visual indent
259:9: E128 continuation line under-indented for visual indent
260:9: E128 continuation line under-indented for visual indent
261:29: E127 continuation line over-indented for visual indent
272:9: E128 continuation line under-indented for visual indent
273:9: E128 continuation line under-indented for visual indent
274:9: E128 continuation line under-indented for visual indent