Adding trkmht algorithm to master
This MR adds the trkmht algorithm into master.
As this introduces a new chain, it also changes the output of REGTESTs so new references are included as well. With these, runTrigART.py -m -j4
runs successfully.
This is also a test case for the new MET trigger coding style introduced by !26533 (merged). If this goes (relatively) smoothly, the other algorithms already present in master will also be migrated to this style.