Adapt PF algorithm for high pT jet substructure

Change in PFAlgo to fix the jet mass reconstruction of highly boosted W bosons.
The PFAlgo creates merged photons and neutral hadrons from an excess of energy in ECAL and HCAL.
With this fix, these neutrals are split up into ECAL clusters to enable jet mass measurement.
The change is only applied to merged neutrals containing >=2 ECAL clusters with E>50 GeV and has therefore no effect on low energy reconstruction.

This change increases slightly the number of high momentum neutral particles in the event, but has no impact on jet energy fractions or jet energy scale/resolution. Only jet mass scale/resolution is improved.

Presentations at xPOG and JetMET
https://indico.cern.ch/event/320480/contribution/8
https://indico.cern.ch/event/322766/contribution/5

This part of the code (all push_backs) is essentially repeated below 6 times. Please make the code more "modular" or restructure it (don't repeat it).