Merged requested to merge atlas-physics/pmg/mcexperts/powheg-experts/athena:issue_13 into 21.6
TWiki has become unwieldy to keep in sync with the PowhegControl source. Following the successful examples of Rivet in ATLAS and other PMG tools, we have migrated the ATLAS Powheg usage instructions and Athena interface documentation to Git.
This MR also contains first updates of the documentation, updating usage instructions to Release 21 and the newer
Gen_tf.py transform used for event generation.