Migrating HLTMonitoring to athenaMT [ATR-19976]

This is the first code for the MT-migrated TrigHLTMonitoring package, also discussed in Jira ATR-19976.

The changes include the files for a basic skeleton of the top-level HLT monitoring. Some MT functionalities have been moved from the old code, to maintain clarity between the old legacy code and the new MT code. This means a few changes in Control/AthenaMonitoring/python/AthenaMonitoringCfg.py and Control/AthenaMonitoring/python/DQConfigFlags.py

Current code is mostly a basic skeleton with a few placeholder histograms, which can be run with the driver script (after normal set-up, check-out and asetup master,latest,Athena, see https://twiki.cern.ch/twiki/bin/view/Atlas/HLTMonMigrationOfT0CodeToAthenaMT#Get_started_with_the_example_cod )

Run3DQTestingDriver.py --dqOffByDefault Input.Files="['/afs/cern.ch/user/e/ebergeas/work/public/triggermonitoring/nigtly_2019-08-24T2130/AOD.pool.root']" DQ.Steering.doHLTMon=True > mylog.txt &

The output currently contains some dummy histograms in the HLT/ResultMon folder. The main change is that the top-level streeing has been moved to where it should be.

This MR did cause conflicts with
!26658 (merged) (HLTMuon)
!26829 (merged) (HLTCalo)
which have been resolved. It is important that this MR is merged as soon as possible.

Tagging @ponyisi @cburton @tamartin @fwinkl @dzanzi @morgens