release/21.6.4

Ewelina Maria Lobodzinska

@ewelina

357e7762 · Merge branch '21.6-jetrec' into '21.6' · Mar 28, 2019

release containing:
Powheg ATLASOTF-00-04-00
and interface fixes for this version (ttbb process added)
some ART tests fixes
RHadron fixes
ATLASJT-392 "21.6 ": Fixes incorrect ordering of ListOfStrings after assignment through job options