21.6 add prophecy4f and hto4l
Migrating Prophecy4f and Hto4l from release 19 to 21.6.
For things to work fully, one will need to migrate changes for externals in: this tag to be used in 21.6 so that the external packages for prophecy4f and Hto4l can be picked up from LCG.
But I have tested bypassing this with:
asetup 21.6,AthGeneration,latest --envvarsNotSet=LCG_RELEASE_BASE
Note also that there is an outstanding bug report here, where is appears that Pythia is changing the decayed four leptons 1% of the time which affects the reconstructed mass. This will need to be resolved before any production might be done. But this shouldn't affect adding this MR into the nightly.