BibIndex: author and parent fields mapping

* New 'parent' field associated with tag '786__w'
* Associates 'author' field with tag '786__w'
* Associates 'exactauthor' field with tag '786__w'
* Adds tags '400__a' and '880__a' to 'exactauthor' field

Signed-off-by: Federico Poli <federico.poli@cern.ch>