ignore 1 exit code in EBLCombine

warning like

    ... extra bytes at beginning or within zipfile
    (attempting to process anyway)

will exit with 1, even though the processing finished
successfully; so ignore 1 exit code in EBLCombine for now
added task to get GND into cayley/leveldb
added hmt workflows stub

this workflow module should hold data reconciliation tasks
using the following sources:

* raw data: bnf, imslp, naxos and rism
* authority data: gnd
added benchmark decorator
use ClosestDateParameter with a weekly schedule