Revert "BibConvert: record type guessing removal"

This reverts commit f42783adf9aaca23cadac8bf776f08d8b1264c95.
BibHarvest: arXiv harvesting improvements

* No longer produces duplicate fields when filtering records.

* Adds special check to remove 980 Thesis or ConferencePaper for
  updates.

* PEP updates.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>