sequtils: new seq generator for texkeys

• Removes special utf-8 characters from author names

• Uses field 502__d for date.

• Adds corporate author.

• If no first author or collaboration found, looks in field 100__a for the first part of the texkey/

• Adds possibility to run texkey generation as a periodic bibtask.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>