Incorporate MET into RecoSteering using run 3 configuration

This MR (finally) corresponds to a version of the new MET configuration that works in RecoSteering. I have tested running "RecoSteeringTest.py --RAW" and this runs ok. Some adjustments were needed to the ReadHandleKey initialisation in METSoftTool. More work will be needed to validate the results. In addition, due to some issue with Tau flow elements these need to be turned off to run. This was done by changing the default value in the legacy configuration, but this change has been reverted in the MR to avoid breaking anything in RecExCommon. Next steps will be to validate the results, and try to ensure the new configuration picks up some of the hardcoded values in a better way.